ACI 224R-1990

ACI 224R-90Control of Cracking in Concrete Structures Reported by ACI Committee 224 The principal causes of cracking in concrete and recommended crack control procedures are presented. The current state of knowledge in microcracking and fracture mechanics is discussed. The control of cracking due to drying shrinkage and crack control for flexural members, layered systems and mass concrete are covered in detail. Longterm effects on cracking are considered, and crack control procedures used in construction are presented. Information is provided to assist the engineer and the constructor in developing practical and effective crack control programs for concrete structures. Keywords: adiabatic conditions; aggregates: air entrainment; anchorage (structural); beams (supports); bridge decks; cement-aggregate reactions; cement content; cement types; compressive strength: computers; concrete construction; concrete pavements; concrete slabs; concretes; conductivity: consolidation; cooling; crack propagation; cracking (fracturing); crack width and spacing: creep properties; diffusivity; drying shrinkage; end blocks; expansive cement concretes; extensibility; failure; fibers; heat of hydration; insulation; joints (junctions); machine bases; mass concrete; microcracking; mix proportioning; modulus of elasticity; moisture content; Poisson ratio; polymer-portland cement concrete; pozzolans; prestressed concrete; reinforced concrete; reinforcing steels; restraints; shrinkage: specifications; specific heat; strain gages; strains; stresses; structural design; temperature; temperature rise (in concrete); tensile stress; tension; thermal expansion; volume change. Contents Chapter 1 - Introduction, page 224R-2 Chapter 2 - Crack mechanisms in concrete, page 224R-2 2.1 - Introduction 2.2 - Microcracking 2.3 - Fracture Chapter 3 - Control of cracking due to drying shrinkage, page 224R-9 3.1 3.2 3.3 3.4 3.5 3.6 Introduction Crack formation Drying shrinkage Factors influencing drying shrinkage Control of shrinkage cracking Shrinkage-compensating concretes Chapter 4 - Control of cracking in flexural members, page 224R-16 Introduction Crack control equations for reinforced concrete beams Crack control in two-way slabs and plates Tolerable crack widths versus exposure conditions in reinforced concrete 4.5 - Flexural cracking in prestressed concrete 4.6 - Anchorage zone cracking in prestressed concrete 4.7 - Tension cracking 4.1 4.2 4.3 4.4 - ACI Committee Reports, Guides, Standard Practices , and Commentaries are Intended for guidance in designing, planning, executing, or inspecting construction, and in preparing specifications Reference to these documents shall not be made in the Project Documents. If items foun d in these documents are desired to be part of the Project Documents, they should be phrased in mandatory language and incorporated into the Project Documents. Cbapter 5 - Long-term effects on cracking, page 224R-21 5.1 5.2 5.3 5.4 5.5 Introduction Effects of long-term loading Environmental effects Aggregate and other effects Use of polymers in improving cracking characteristics Copyright 0 1990 , American Concrete Institute. All rights reserved including rights of reproduction and use in any form or by any means, including the making of copies by any photo process, or by any electronic or mechanical device, printed or written or oral, or recording for sound or visual reproduction or for use in any knowledge or retrieval system or device, unless permission in writing is obtained from the copyright proprietors. 224R-1 ACI COMMITTEE REPORT Chapter 6 - Control of cracking in concrete layered systems, page 224R-23 6.1 6.2 6.3 6.4 Introduction Fiber reinforced concrete (FRC) overlays Latex modified concrete (LMC) overlays Polymer impregnated concrete (PIC) systems Chapter 7 - Control of cracking in mass concrete, page 224R-26 7.1 7.2 7.3 7.4 7.5 7.6 7.7 Introduction Crack resistance Determination of temperatures and tensile strains Control of cracking Testing methods and typical data Artificial cooling by embedded pipe systems Summary - Basic considerations for construction controls and specifications Chapter 8 - Control of cracking by correct construction practices, page 224R-36 8.1 8.2 8.3 8.4 8.5 8.6 8.7 Introduction Restraint Shrinkage Settlement Construction Specifications to minimize drying shrinkage Conclusion inforced and prestressed concrete members have been condensed into a single chapter, Chapter 4, on crack control in flexural members. The resulting presentation is more concise and, hopefully, more useful to the structural designer. Chapter 5, on long-term effects, details some interesting findings on the change of crack width with time. Chapters 3, 7, and 8, which consider drying shrinkage, mass concrete, and construction practices, respectively, have been expanded and updated to take into account the most recently developed procedures in these areas. In addition, new sections have been added to Chapters 7 and 8 which provide specific guidance for the development of crack control programs and specifications. The committee hopes that this report will serve as a useful reference to the causes of cracking and as a key tool in the development of practical crack control procedures in both the design and the construction of concrete structures. References 1.1. ACI Committee 224, “Control of Cracking in Concrete Structures,” ACI JOURNAL, Proceedings V. 69, N O . 12, Dec. 1972, pp. 717-753. 1.2. ACI Committee 224, “Causes, Mechanism, and Control of Cracking in Concrete,” ACI Bibliography No. 9, American Concrete Institute, Detroit, 1971, 92 pp. Chapter 9 - References, page 224R-42 9.1- Specified and/or recommended references 9.2 - Cited references Chapter 1 - Introduction Cracks in concrete structures can indicate major structural problems and can mar the appearance of monolithic construction. They can expose reinforcing steel to oxygen and moisture and make the steel more susceptible to corrosion. While the specific causes of cracking are manifold, cracks are normally caused by stresses that develop in concrete due to the restraint of volumetric change or to loads which are applied to the structure. Within each of these categories there are a number of factors at work. A successful crack control program must recognize these factors and deal with each of them, in turn. This report presents the principal causes of cracking and a detailed discussion of crack control procedures. The body of the report consists of seven chapters designed to help the engineer and the contractor in the development of effective crack control measures. This report is an update of a previous committee report, issued in 1972.1.1 The original report was supplemented by an ACI Bibliography on cracking,1 . 2 also issued by this committee. In the updating process, many portions of the report have undergone sizeable revision, and the entire document has been subjected to a detailed editorial review. Chapter 2, on crack mechanisms, has been completely rewritten to take into account the experimental and analytical work that has been done since the completion of the first committee report. Chapter 6, on crack control in concrete layered systems, is new to the report and deals with a form of concrete construction that was in its infancy at the time the first report was drafted. Individual chapters on crack control in re- Chapter 2 - Crack mechanisms in concrete* 2.1 - Introduction Beginning with the work at Cornell University in the early 1960s,2 .1 a great deal has been learned about the crack mechanisms in concrete, both at the microscopic and the macroscopic level. Of special interest during the early work was the realization that the behavior of concrete, under compressive as well as tensile loads, was closely related to the formation of cracks. Under increasing compressive stress, microscopic cracks (or microcracks) form at the mortarcoarse aggregate boundary and propagate through the surrounding mortar, as shown in Fig. 2.1. During the first decade of research, a picture developed that closely linked formation and propagation of these microcracks to the load-deformation behavior of concrete. Prior to load, volume changes in cement paste cause interfacial cracks to form at the mortar-coarse aggregate boundary.2.2,2.3 Under shortterm compressive load, no additional cracks form until the load reaches approximately 30 percent of the compressive strength of the concrete.2.1 Above this value, additional bond cracks initiate throughout the matrix. Bond cracking increases until the load reaches approximately 70 percent of the compressive strength, at which time microcracks begin to propagate through the mortar. Mortar cracking continues at an accelerated rate until the material ultimately fails. For concrete in uniaxial tension, experimental work indicates that major microcracking begins at about 60 percent of the ultimate tensile strength.2.4 ‘Principal author: David Darwin. CONTROL OF CRACKING 224R-3 Studies of the stress-strain behavior and volume change of concrete 2.5 indicate that the initiation of major mortar cracking corresponds with an observed increase in the Poisson’s ratio of concrete. The term “discontinuity stress” is used for the stress at which this change in material behavior occurs. In general, it has been agreed that the microcracking that occurs prior to loading has very little effect on the strength of concrete. However, work by Brooks and Neville 2.6 indicates that the effect of early volume change on microcracking of concrete may result in a reduction of both tensile and compressive strength as concrete dries out. Their study shows that upon drying, the strength of test specimens first increases and then decreases. They postulate that the initial increase is due to the increased strength of the drier cement paste and that the ultimate decrease in strength is due to the formation of shrinkage induced microcracks. Work by Meyers, Slate, and Winter 2.7 and Shah and Chandra2.8 demonstrates that microcracks increase under the effect of sustained and cyclic loading. Their work indicates that the total amount of microcracking is a function of the total compressive strain in the concrete and is independent of the method in which the strain is applied. Sturman, Shah, and Winter2.9 found that the total degree of microcracking is decreased and the total strain capacity in compression is increased when concrete is subjected to a strain gradient. At about the same time that the microcracking studies began, investigators began applying fracture mechanics to the studies of concrete under load. The field of fracture mechanics, originated by Griffith2.10 in 1920, serves as the primary tool for the study of brittle fracture and fatigue in metal structures. Since concrete has for many years been considered a brittle material in tension, fracture mechanics is considered to be a potentially useful analysis tool for concrete by many investigators. 2. .12 The field of fracture mechanics was first applied to concrete by Kaplan 2.11 in 1961. The classical theory serves to predict, the rapid propagation of a macrocrack through a homogeneous, isotropic, elastic material. The theory makes use of the stress intensity factor, KI , which is a function of crack geometry and stress. Failure occurs when KI reaches a critical value, K Ic , known as the critical stress-intensity factor under conditions of plane strain. KIc is thus a measure of the fracture toughness of the material. To properly measure KIc for a material, the test specimen must be of sufficient size to insure maximum constraint (plane strain) at the tip of the crack. For linear elastic fracture mechanics (LEFM) to be applicable, the value of KIc must be a material constant, independent of the specimen geometry (as are other material constants such as yield strength). The earliest experimental work utilized notched tension and beam specimens of mortar and con- $EGlrgyj m 0.0012 STRAIN m STRAIN 0. CKI Fig. 2.1 - Cracking maps and stress-strain curves for concrete loaded in uniaxial compression. * *From S. P. Shah, and F. O. Slate, “Internal Microcracking, Mortar-Aggregate Bond and the Stress-Strain Curve of Concrete,” Proceedings, International Conference on the Structure of Concrete (London, Sept. 1965), Cement and Concrete Association, London, 1968, pp. 82-92. crete. 2.11-2.14 The crack resistance was expressed in terms of the strain energy release rate at the onset of rapid crack growth, G, which is directly related to the fracture toughness of the material. Later investigations evaluated the crack resistance of paste, mortar and concrete in terms of the fracture toughness, itself.2.15 Work by Naus and Lott2.16 indicated that the fracture toughness of paste and mortar increased with decreasing water-cement ratio, but that the water-cement ratio had little effect on the fracture toughness of concrete. They found that KIc increased with age, and decreased with increasing air content for paste, mortar, and concrete. The effective fracture toughness of mortar increased with increasing sand content, and the fracture toughness of concrete increased with an increase in the maximum size of coarse aggregate. Additional work by Naus,2.17 presented just prior to the previous committee report,1.1 indicated that fracture toughness was not independent of specimen geometry for tensile specimens of paste, mortar and concrete and that fracture toughness was a function of the crack length. These observations lead to the possibly erroneous conclusion that fracture mechanics may not be applicable to concrete. Because certain size requirements must be met, before fracture mechanics is applicable, these results may only indicate that the test specimen did not satisfy all of the minimum size requirements of linear elastic fracture mechanics. The balance of this chapter describes some of the more recent studies of crack mechanisms in concrete and gives a somewhat different picture from that presented in the previous committee report. 224R-5 l/3 0 w OUNCOATEDAGG. He attributed the increase in strength to the effect of the polymer on the strength of mortar. seemed to indicate a very large effect. thus emphasizing the importance of interfacial strength on the behavior of concrete.” presented at the First International Conference on Mathematical Modeling (St. - z 10 oz 1600 2000 MICROSTRAIN 2400 2800 3200 Fig. They also observed a 10 percent reduction in the compressive strength for specimens with low mortar-aggregate bond strength. 2. “Microscopic Finite Element Model of Concrete. but did increase the compressive strength of concrete. but has very little effect on the discontinuity stress. Darwin. These studies utilized relatively thick. 2.31. l COATED AGG. soft coatings on the coarse aggregate to reduce the bond strength. Their results indicate that reducing the interfacial strength of the aggregate decreases the initiation stress by about 20 percent. 3 7 used glass spheres with different degrees of surface roughness as coarse aggregate. Maher. . 3 7 which did not isolate the coarse aggregate from the mortar indicate that the interfacial strength plays only a minor role in controlling the stress-strain behavior and ultimate strength of concrete. Carino found that polymer impregnation did not increase the interfacial bond strength. 2 . Perry and Gillott 2 . 3 6 .37 and Maher and Darwin. the effect was more like inducing a large number of voids in the concrete matrix. Darwin and Slate 2 . seems to corroborate these two studies.4). and D. * *From A.Stress-strain curves as influenced by coating aggregates (Reference 2.-Sept. the average amount of mortar cracking was slightly greater for the specimens made with coated aggregate. 2.Stress-strain curves for concrete model.32 Using a linear finite element representation of a physical model of concrete. They found that a large reduction in interfacial bond strength causes no change in the initial stiffness of concrete under short-term compressive loads and results in approximately a 10 percent reduction in the compressive strength as compared to similar concrete made with aggregate with normal interfacial strength (see Fig. thus downgrading the importance of the interfacial bond. Louis. However. Since these soft coatings isolated the aggregate from the surrounding mortar. Work by Carino.2.38 using polymer impregnated concrete. and ultimately cement paste. 2.36). in every case. 3 6 used a thin coating of polystyrene on natural coarse aggregate. 1 9 7 7 ) .4 . in controlling the stress-strain behavior of concrete is illustrated by the finite element work of Buyukozturk2. The importance of mortar. Two other s t u d i e s 2 . This small yet consistent difference may explain the differences in the stressstrain curves.5 . They also found that the lower interfacial strength had no appreciable effect on the total amount of microcracking. Buyukozturk was able to simulate the overall crack patterns under uniaxial loading.2. Mortar Fig. Aug. 25 While the statistical variations undoubtedly play a part.6 . The results for Buyukozturk’s model are shown in Fig.32 Fig.2.Stress-strain curve for finite element model of concrete with varying values of mortar-aggregate bond strength (Reference 2. In every case the failure of the model is governed by “crushing” of the mortar which occurs at an average strength below that of the mortar alone. 2. They also observe that the stress at discontinuity occurs at approximately 75 percent of the ultimate strength in compression and at about 60 percent of the ultimate strength for those cases involving tension. et al.33. PSI ACI COMMl=lTEE REPORT 4 lMPar w\ ..7 . a very close representation of the actual behavior can be obtained.2. 0. 2. However. The dominant effect of microcracking is to increase the lateral strain. 0. 2. and Nilson 2. or above normal. Shah and McGarry also ran a series of tests using notched tensile specimens and determined that paste speci- ti * OS NOTCH DEPTH. 0 Strdln.2.)/3. but has no significant effect on strength.40 lhave observed that the principal tensile strain in concrete at the “discontinuity stress” appears to be a function of the mean normal stress.3. 2. 2. his finite element model could not duplicate the nonlinear experimental behavior of the physical model using the formation of interfacial bond cracks and mortar cracks as the only nonlinear effect.5 s and Tasuji.8..2.4) (12. 1 (6. = (0.. Newman 2.42).2.+0. 0 Fig. a number of investigations have shed additional light on the applicability of fracture mechanics to concrete and its constituent materials. For cyclic and sustained loading.41 mortar cracking itself controls the strength of concrete or whether it only signals intimate damage of the cement paste remains to be seen.42 Their work indicates that while paste is notch sensitive. Slate.224R-6 S t r e s s .31. amount of microcracking. A number of investigators feel that the onset of mortar cracking marks the “true” ultil Whether mate strength of concrete.2.7)(19. matching the levels at which mortar cracking begins.” When the proper statistical variation is selected.34. In normal weight concrete.6-2.Effect of notch depth on flexure strength (Reference 2.3 . neither mortar nor concrete are affected by a notch (Fig.?.32 have shown that by using a nonlinear representation for the mortar constituent of the physical model. the damage to cement paste seems to play an important role in controlling the primary stress-strain behavior of concrete under short-term axial load.39 to duplicate the nonlinear behavior of concrete utilizing microcracking alone has been explained as being due to the fact that concrete is really a “statistical material. Overall. Maher and Darwin2 . The stressstrain curve for the model without cracking differs very little from that of models that have a normal.32).25. increasing the initial stiffness and decreasing the strength of the paste. 2.4 l Their work seems to point very strongly toward a “limiting tensile strain” as the governing factor in the strength of concrete.5 . .2.26. aggregate particles act as stress-raisers. _ ._ 1 *2. Microcracks have a relatively minor effect on the primary stress-strain behavior of the models. Additional studies in this area are clearly warranted. the nonlinear behavior of concrete can be v MORTAR@21 v 0 CONCRETE 0 duplicated 2.2. .Normal fnterfaclal Str. 0 0.Fracture Since the publication of the previous report.31. observe that the final failure of their specimens consists of the formation of macroscopic tensile cracks. 2.001 In/in 1. Shah and McGarry utilized flexure specimens subjected to three-point loading. In their study of the biaxial strength of concrete.’ -..0 . 2.7)._ I nfuxte I nterfaciaf Str Zwo Tenstle and Cohewe I nterfaclal Str. 0 L_. 1)(2X 4) I I I I lJ4 l/2 314 I * .6 illustrates the results obtained for a highly simplified model of concrete under uniaxial compression using a nonlinear representation for mortar.+0._ Aggregate Mortar *l. the major nonlinear behavior can also be matched by considering the nonlinearities of the mortar constituent. a great deal of the bond cracking results from load induced volume changes within the paste. The inability of linear elastic models 2. . INCHES (mm) Fig.2. Tasuji.Z e r o InterfacIal Str.2. Mindess and Nadeau investigated the effect of notch width on KI for both mortar and concrete. . for valid fracture mechanics test results. These size requirements are dependent upon the square of the toughness levels being measured. the test specimens must meet minimum size requirements (ASTM E 399). Since their work utilized small specimens with a depth of only about 50 mm (2 in.). 2. larger test specimens must be used with tougher materials such as mortar. 2. they found that within the range studied. Because the plane strain-plane stress transition occurs beyond the limits of LEFM. Arnesen2. the specimen capacity is governed by the modulus of rupture of concrete. calculated from the linear stress distribution. 2. indicating that the addition of fine aggregate improves the toughness of paste.23 investigated the 0.Relationship bet ween test results and theory for notched concrete beams (Reference 2. Brown utilized notched flexure specimens and double cantilever beam specimens of paste and mortar2. The applicability of these results. the analysis is more complex. the strength is governed by the fracture toughness. but less sensitive than cement paste.-~ 1 Fig.42 As shown in Fig.20 Utilizing notched beam specimens of constant length and depth. must have specimen dimensions four times that of the first material for the test results to be equally valid. are notch sensitive.Effect of notch depth on flexural strength (Reference 2.18 8 His tests show that the fracture toughness of cement paste is independent of crack length and is therefore a material constant. In separate investigations of notched beam specimens2.9.21 ’ and beams with right angle re-entrant notches2..). 2. This type of behavior is also observed in metals.e.CONTROL OF CRACKING 224R-7 mens. for valid toughness testing of concrete.22 Walsh has demonstrated that specimen size has a marked influence on the applicability of linear elastic fracture mechanics to the failure of plain concrete specimens.22). Walsh concluded that. with varying widths.10 L I I 1 a/a0 (log scale) 4 Fig.2. The fracture toughness of mortar. and much of the other fracture mechanics work. the depth of notched beams must be at least 230 mm (9 in.~_. Gjorv. They feel that linear elastic fracture mechanics is applicable to the small specimens of . As illustrated in Fig. L-__ -. they were simply measuring the modulus of rupture. mortar and concrete using three-point bend specimens similar to those used by Shah and McGarry2. however. they determined that both mortar and concrete are notch sensitive. there is some indication that rather than measuring the fracture toughness of the material.8 . notch sensitivity of paste. for specimens of similar geometry but below a certain critical size. i.8. Thus a material whose toughness level is twice that of another material (all other properties being equal). has been brought into perspective based on the experimental work by Walsh. Sorensen and. To re-establish the applicability of LEFM. and mortar specimens made with fine aggregate that passed the #30 sieve. there was no dependence of fracture toughness upon the length of crack front. increases as the crack propagates. but that mortar specimens containing larger sizes of aggregate are not notch sensitive. which he approximated as a function of the square root of the compressive strength of the concrete. They conclude that the disagreement with the earlier results is due in part to their improvement in the loading procedure. This behavior is similar to the behavior found in structural steels that exhibit a plane strain-plane stress transition.23). For specimens above this size.9 . Special Report No. Joseph. H.. 1970. 1969. George. Japan.15.13.. Clyde E. 60. No.” ACI JOURNAL . Proceedings V. Sendai.. pp. Slate. A. Floyd O.” Proceedings. Naus. A. pp. Jan. however. 2. EM3. 1963.. “Measuring the Fracture Toughness of Cement Paste and Mortar.. “Microcracking and Stress-Strain Curves for Concrete in Tension.18.” ACI JOURNAL . Dec. 2. pp. No. and proportions of the mix. but that it is not applicable to small specimens. 60-68. 61-64.2. 5. V. 2. Proceedings V. Lott.7. pp.. which shows a combination of both aggregate fracture and bond failure. Dan J. Highway Research Board.2. Surendra P...24 used compliance measurement to monitor crack growth in notched concrete beams subjected to sinusodial loading.16. 58. Jan. 81.185-196.4.17. ASCE. University of Colorado. Floyd O. Gerald M. International Conference on the Structure of Concrete (London. F. and Lott. 2. No. London.” MSc Thesis. June 1963. 24. Research and Testing (Paris).22 that show that if the specimens are large enough. 2.” Proceedings. 1977. No....C. C. specimen size requirements must be given more attention. Proceedings V. A number of investigators do not feel that the Griffith theory of linear fracture mechanics is directly applicable to all concrete2. pp. No. “Microcracking of Plain Concrete and the Shape of the Stress-Strain Curve. J.12. R. and Jones2. 1961.. J. and Winter. “Effects of Flexural Strain Gradients on Microcracking and Stress-Strain Behavior of Concrete. 2. and Batson.24 feel that the theory has application when the limitations and specific nonhomogenous effects are taken into account. 221A. and Winter. 60. Gerald M. 2. Proceedings V. 90. “Fracture Toughness of Portland Cement Concretes. First International Conference on Fracture. 2. aggregate and the paste-aggregate interface. et a1.. “Criteria for the Behavior of Plain Concrete Under Complex States of Stress. Sept. 67. No.. 163-198. 1965. George. James P. Brooks. 62. Shah. “Crack Propagation in Plain Concrete. and that the characteristic value of KIC for aggregate is approximately ten times that of paste. and if the calculation considers the nonlinear material response of concrete.” Materials and Structures. have some degree of notch sensitivity since the failure is not consistent with the modulus of rupture based on the net cross section.. Boulder. pp. 10. 100. 2. and Chandra. J. 147-168. 1343-1382.” PhD Thesis.. 66.” ACI JOURNAL . S. “Volume Changes on Setting and Curing of Cement Paste and Concrete from Zero to Seven Days.14. 209-224. Oct.. Surendra P. 481-489. A.. 1920. 816-824.10. 591-610.24* 2.” ACI JOURNAL. Proceedings V.42 (ASTM E 399). Even the small specimens of mortar and concrete. No. They feel that. “Mechanics of Crack Arrest in Concrete. pp. H. pp. Huang.23. 805-822. compact tension specimens to measure the fracture toughness of paste.21 they agree that LEFM is applicable to large concrete specimens.” Symposium on Structure of Portland Cement Paste and Concrete.9. Nov.” Transactions. 2. Meyers. 1965). Hu. “Crack Propagation Studies in Microconcrete. fracture mechanics does offer an important tool for evaluating the constituent materials of concrete. “Applicability of Linear-Elastic Fracture Mechanics to Portland Cement Concretes. 7.. but not to the small size specimens of mortar and concrete. Elasticity and Strength of Concrete in Tension and in Compression. pp. 1. 6. Gordon B. pp. Floyd O. Clearly. “Crack Propagation and the Fracture of Concrete. No.” Magazine of Concrete Research (London). June 1969. pp.5.43 utilized wedge loaded. “Relationship Between Time-Dependent Deformation and Microcracking of Plain Concrete.” ACI JO U R N A L. V.. Newman. 2.. 255-274. Naus. 1968.11. 2. 131-141. 1967. James L. M. 2. Slate. Sept. Their work indicates that the KIc values for interfacial strength between paste and aggregate is only about one-third of the KIc value for paste alone. “A Comparison of Creep. V.3.ACI COMMITTEE REPORT paste. “Static and Fatigue Fractures of Portland Cement Mortars in Flexure. and Kesler.. Cement and Concrete Association. Evans. 1. pp. Urbana.. Proceedings V. Sturman.. Of key interest in future work are the observations by Walsh2. S. Thomas T. and Marathe. T. 1971. Washington.1. 1968. Thomas. .” ACI J OURNAL Proceedings V. Brown. Hsu. George. Hsu. 2. while the failure of concrete in tension and compression is controlled by many interacting cracks rather than by the propagation of a single crack. 1. “The Phenomena of Rupture and Flow in Solids. 2. Bernard L. pp. July 1965. 66.6. Feb. References 2. V.” ACI JOURNAL . Mar.. Based on the appearance of the fracture surface. 2. 371-390. No.. James L. 89. M. Swartz. 204-218. No. Kenneth. Shah. V.2. T. 1. pp. Some like Swartz. D. 64. Proceedings V. Jan. C.” Magazine of Concrete Research (London). 2. However. 29. 2. 34-39. they state that an “effective” fracture toughness might be a significant material property if related to specific material and specimen variables such as aggregate size and gradation. 1972. University of Illinois. Glucklich. “Fracture of Concrete Subjected to Cyclic and Sustained Loading. Ramon E. Department of Civil Engineering. 1966.. they feel that fracture toughness is not a pertinent material property. They found that paste is a notch sensitive material and that the addition of entrained air or soft particles has only a small affect on K I c .8. No. 1963.” ACI JOURNAL. Citing Walsh’s earlier work. Slate. Griffith. No. 1966. Sushil. M. and Matheus.21’ 2. 3.” Proceedings. and Winter. Kaplan. “Mathematical Analysis of Shrinkage Stresses in a Model of Hardened Concrete. 2. Hillemeier and Hilsdorf 2. Dan J. pp. pp. They conclude that this procedure is useful for monitoring crack growth in concrete due to fatigue. Romualdi. Aug. and Neville. Royal Society of London. the effects of heterogeneity are greatly reduced and that concrete may approximate a homogenous material to which the principles of fracture mechanics can be applied. Sturman. 2. 2. 2. Pomeroy. 1976. Testa and Norris Stubbs. Nov. 523-535. 333-344. 1976. Norris. Carino. v. 7. and Darwin. Mindess. “Mechanism of Cyclic Creep of Concrete. 2. Kuo-Kuang. “Griffith Fracture Criterion and Concrete.. James 0. Although drying shrinkage is one of the principal causes of cracking.. 7.41. 2.. “Microscopic Finite Element Model of Concrete. David. Karsan.” Proceedings.23. and Stubbs.” Magazine of Concrete Research (London). 83-101. V.. C. 1969. V. Shah. EM4. pp. engineers. American Concrete Institute. Mar. D. A. Cornell University. F. Good design and construction practice can minimize the amount of cracking and eliminate the visible large cracks by the use of adequate reinforcement and contraction joints.30. 69-72. ASCE. V. No. 770-781. G. 103. University of Missouri-Rolla. 95... Cracking may also develop in the concrete prior to hardening due to plastic shrinkage.34. pp. ASCE. 1978.22. pp.. 1978.” by Rene B. David. 529-534.” Cement and Concrete Research. Hu. I.” Magazine of Concrete Research (London).. V. Walsh. and Arnesen. No. pp. ST12. C. Sept.” Douglas McHenry Symposium on Concrete and Concrete Structures.28. “Effect of Notch Width of KIC for Mortar and Concrete.36. Proceedings V . 7. chemical reactions. and’ Slate. and the use of expansive cements to minimize cracking.40. No. pp. Discussion of “Bond Failure and Inelastic Response of Concrete. 2. 1977. No.. 46. pp. A. 553-564. pp. and Hilsdorf. Ithaca. “Effect of PasteAggregate Bond Strength on Behavior Concrete.25. Sorensen. 2. “A Quantitative Assessment of Damage Sustained in Concrete During Compressive Loading. “Bond Failure and Inelastic Response of Concrete. Arthur H. “Inelastic Behavior and Fracture of Concrete. 469-470. O. 1705-1714. 789-800. “Modal Determination of the Effect of Bond Between Coarse Aggregate a n d M o r t a r o n t h e C o m p r e s s i v e Strength of Concrete. P. and Chandra. E. K. No.35. David.” Proceedings. 9. are frequently responsible for cracking of hardened concrete.. 11. pp.. George. Gary L. 1971.37. Buyukozturk. 306-312. 65. Rene B. 2.. 5.1 . Tasuju. 1970. pp. 6.. 476. V. EM6. Surendra P. Floyd 0.-Sept.” ACI JO U R N A L.. F. pp.Introduction Cracking of concrete due to drying shrinkage is a subject which has received more attention by architects. “The Influence of Mortar-Aggregate Bond Strength on the Behavior of Concrete in Uniaxial Compression. Surendra P. Apr. 2543-2563. Sushil. No. 2. 2. 2. ASCE. June 1972.” ACI JO U R N A L .6) of this report. Shah. Sept. Slate. 1977). III. David. 104. and Nilson. and Anderson. s Polivka.31. V. A. Evans. 6. V. and Jones. Darwin. 2.19. Sept.. “Stress-Strain Response and Fracture of Concrete in Biaxial Loading. 94. ASCE. J. No. Apr. 104. Perry..29. Maher. 151-160. B. Dec. pp. 2. pp. Maher. pp. and Dougill. Nov. 97. V. 2. Information presented in this chapter concerns only the subjects of cracking of hardened concrete due to drying shrinkage. SP-55.. John S. 28.” Cement and Concrete Research. and Darwin. 1663-1676. “A Finite Element Model to Study the Microscopic Behavior of Plain Concrete. “Effects of Polymer Impregnation on Mortar-Aggregate Bond Strength. 1968.” ACI JO U R N A L . “Fracture Mechanics Studies of Concrete Compounds.” Indian Concrete Journal (Bombay). 21-29. V. First International Conference on Mathematical Modeling (St. 37-41. Sept. 24. and contractors than any other characteristic or property of concrete. and Microcracking of Concrete. V. 1. O.21. July 1976. M.. J. F. Gjorv. Detroit. 1969. No. C. H. and Winter. pp. No.38. Ataullah. V. D. and Jirsa. Mar. Nicholas J. pp. S. “The Stress-Strain Relationship for Hardened Cement Pastes in Compression. factors influencing shrinkage. Sidney. No. Surendra P. J. as well as excessive tensile stresses due to loads on the structure. Sept. 66.. M. 3. 28. July 1977. “Fracture of Plain Concrete. pp. The subject of construction practices and specifications to minimize drying shrinkage is covered in Chapter 8 (Sections 8.” J o u r nal of Materials. E. Proceedings V. V. Volume Change. July 1976. 1977. and Nadeau. Dec. Spooner. No.” Proceedings . pp. 94. Testa. H. 1978.. 2. 4. “Compliance Monitoring of Crack Growth in Concrete. 7. “Crack Initiation in Plain Concrete. pp.” PhD Thesis. No. and Hirst. W. 75. and McGarry. I. It is one of the most serious problems encountered in concrete construction. Darwin. W. Jan. 2.43. E.33.32. G.24.” Cement and Concrete Research. 85-92.. C. 439-447. “Damage and Energy Dissipation in Cement Pastes in Compression. 1976. Spooner. Aug.42. “Behavior of Concrete under Compressive Loadings.” Cement and Concrete Research. Palle. V. Proceedings V. EM2. frost action.. 9.. Mar.” CRINC Report -SL-76-02. July 1978. 1972. Hillemeier. Demir.20. A. 2.39.. Proceedings. Lawrence. and Dougill. 2. No. 2.. Tommy P. “Notch Sensitivity and Fracture Toughness of Concrete. 92. 79. 1966. May 1977. Ebrahim. pp. Neville. Oral. 5. Chapter 3 . “Critical Stress. 5.” Cement and Concrete Research. 4.. P. D. pp. Walsh. V. Aug. 2. 1. V. Spooner. pp. ASCE. Swartz..” Proceedings. D. and Gillott. EM2. 507-509.CONTROL OF CRACKING 224R-9 2. 925-930. 2. V. temperature stresses. 27. 4. No. “Stress-Strain Response and Fracture of a Model of Concrete in Biaxial Loading. R. The University of Kansas Center for Research. pp. Ataullah. 535-547. J.Control of cracking due to drying shrinkage* 3. V. P r o ceedings V. control of cracking. 86-98.” Proceedings. C. 7... *Principal author: Milos . 63. and Nielsen.27.. Clifton.26. pp. 2. 2. 1975. Nepper-Christensen. 1977.” ACI JOURNAL .. 296-310.. June 1970.3 and 8. 83 pp. No.” Cement and Concrete Research. Louis. Shah. No.. 2.” Magazine of Concrete Research (London). 2. Fred J. 1977. “The Fracture Mechanics of Mortars. Stuart E..” Magazine of Concrete Research (London). In a hardened cement paste. a low modulus of elasticity and high creep characteristics of the concrete are desirable since they reduce the magnitude of tensile stresses. shrinkage due to carbonation is of minor importance in the overall shrinkage of a concrete structure. However. 3. 3. are an inherent characteristic of hydraulic cement concretes. (c) the modulus of elasticity of the concrete. the concrete would not crack.” produce a hydration product consisting essentially of some crystalline materials (principally calcium hydroxide) and a large amount of hardened calcium silicate gel called “tobermorite gel. The magnitude of tensile stress developed during drying of the concrete is influenced by a combination of factors. to minimize cracking. especially at early ages. These volume changes. present in the atmosphere on the hydration products of the cement. but a significant amount is in the tobermorite gel. with changes in moisture content. a large extensibility of a concrete member subjected to bending will cause larger deflections. This combination of shrinkage and restraint develops tensile stresses. Since the drying shrinkage is always larger at the exposed surface. CO2. some of the water is in the capillary pores of the paste.1 . shrinkage. and when it is wetted again.. However. This is illustrated in Fig. the interior portion of the member restrains the shrinkage of the surface concrete. it expands. while the aggregate provides an internal restraint which significantly reduces the magnitude of these volume changes. in a structure the concrete is always subject to some degree of restraint by either the foundation or another part of the structure or by the reinforcing steel embedded in the concrete. It is the change in moisture content of the cement paste that causes the shrinkage or swelling of concrete. Since carbon dioxide does not penetrate deep into the mass of concrete. such as (a) the amount of shrinkage.224R-10 ACI COMMITTEE REPORT 3. It is the loss of the adsorbed and inter-layer water from the hydrated gel that causes the shrinkage of the paste. . several chemical reactions take place.Crack formation Why does concrete crack due to shrinkage? If the shrinkage of concrete caused by drying could take place without any restraint. The action of carbon dioxide. Another type of restraint is developed by the difference in shrinkage at the surface and in the interior of a concrete member. On drying the first water lost is that which occupies the relatively large size capillaries in the cement paste. This may cause surface cracking. (b) the degree of restraint. Thus. thus developing tensile stresses. These reactions.Drying shrinkage When concrete dries. ORIGINAL LENGTH I I UNRESTRAINED SHRINKAGE t- RESTRAINED SHRINKAGE DEVELOPS TENSILE STRESS IF TENSILE STRESS IS GREATER THAN TENSILE STRENGTH. When cement is mixed with water.2 . moisture slowly diffuses from the interior mass of the concrete to the surface where it is lost by evaporation. 3. When a concrete is exposed to drying conditions. CONCRETE CRACKS Fig. the cement paste is also subject to carbonation shrinkage. Thus. principally calcium hydroxide. the amount of shrinkage is only o n e factor governing the cracking.1.” This rigid gel consists of colloidal size particles and has an extremely high surface area. which are cracks that do not penetrate deep into the concrete. Ca(OH)2. This loss of water causes very little. the concrete will crack. which is accompanied by a decrease in volume. However. and (d) the creep or relaxation of the concrete. When this tensile stress reaches the tensile strength. In addition to drying shrinkage. Shrinkage is due to the loss of adsorbed water from the gel. These surface cracks may with time penetrate deeper into the concrete member as the interior portion of the concrete is subject to additional drying. commonly called “hydration.3 .Cracking of concrete due to drying shrinkage. the concrete should have low drying shrinkage characteristics and a high degree of extensibility (low modulus and high creep) as well as a high tensile strength. results in the formation of calcium carbonate. it contracts or shrinks. CaCO. As far as cracking is concerned. causing an expansion of the concrete. if any. On wetting this process is reversed. can vary over a wide range.6 l-year shrinkage. which occupy between 65 and 75 percent of the total concrete volume.116 0. the shrinkage of pastes cured for 28 days was about the same for the two types of cements. Concrete may be considered to consist of a framework of cement paste whose large potential shrinkage is being restrained by the aggregate. Thus.032 Aggregate Sandstone Slate Granite Limestone Quartz t Specific gravity 2. in general. the environment.047 0. higher C4AF contents of the cement. Tests by Carlson3. lower C3A/SO3 ratios. Tests made by the California Division of Highways 3. (b) the bond between paste and aggregate.1 . These and other factors influencing magnitude and rate of shrinkage are herein discussed. Since the rigidity of certain aggregates. feldspar. and some basalts can be generally classified as lowshrinkage producing types of aggregates. . slate. The rate of moisture loss or the shrinkage of a given concrete is greatly influenced by the size and shape of the concrete member.67 2. dolomite.1 Effect of cement .74 2.041 0.6 As an example some of his shrinkage data for concretes with identical cements and identical water-cement ratios are given in Table 3. The absorption of an aggregate.Results of an extensive study made by Blaine. This is true for sandstone and other aggregates of high absorption capacity. and Yudenfreund3. Their results on neat cement pastes showed a wide distribution of shrinkage values especially for the Type I cements. Quartz. limestone or dolomite.3. Cement producers moderate the differences in shrinkage due to cement composition by optimizing its gypsum content.Factors influencing drying shrinkage The major factors influencing shrinkage include the composition of cement. Although the compressibility is the most important single property of aggregate governing concrete shrinkage. The factors which influence the ability of the aggregate particles to restrain shrinkage include (a) the compressibility of aggregate and the extensibility of paste. and 3.8 0. but the increase in shrinkage with increasing fineness is not large. hornblende and some types of basalts. such as expanded shales.3.0 1. Highshrinkage concretes often contain sandstone.2 Influence of type of aggregate . such as granite. which is a measure of porosity. water content. However.4. (c) the degree of cracking of cement paste. and much lower than Type III cements. percent 5.6 showed that finer cements generally result in greater concrete shrinkage. A low modulus is usually associated with high absorption.3.068 0. Of these several factors.2 of the National Bureau of Standards on a large number of portland cements indicate that it is not possible to say that a cement. influences its modulus or compressibility. The subject of shrinkage due to carbonation is discussed in detail by Verbeck. particularly when subjected to long-term exposure to drying. They found that lower shrinkage of pastes was associated with: 1.0015 to more than 0. and Evans.Effect of type of aggregate on shrinkage of concrete3. have a major influence on shrinkage. and mix proportions. Thus. The higher the stiffness or modulus of elasticity of an aggregate. The 6 month drying shrinkage strain of the neat pastes ranged from about 0.CONTROL OF CRACKING 224R-11 carbonation does play an important role in the shrinkage of small laboratory test specimens.3 0 .1. percent 0. because it conforms to the requirements of one of the standard types of cements. and (d) the contraction of the aggregate particles due to drying. their effectiveness in restraining drying shrinkage will vary accordingly. His results show that the composition of the cement is a factor and thus for some cements an increase in fineness may show little change and in some cases even a lower concrete shrinkage.75 2. granite.4 on mortar or paste as a measure of behavior in concrete indicate that Type II cements generally produce lower shrinkage than Type I cements. The fineness of a cement can have some influence on drying shrinkage. Arni. and the time of drying exposure.2 0.5 show that the proportion of gypsum in the cement has a major effect on shrinkage.66 Absorption. The large influence of type of aggregate on drying shrinkage of concrete was shown by Carlson. the more effective it is in reducing the shrinkage of concrete. type of aggregate. Tests by Brunauer.47 2.4. The drying shrinkage of a concrete will be only a fraction (about l/4 to l/6) of that of the cement paste.1 3. aggregate of high modulus of elasticity and low absorption will produce a lowshrinkage concrete.4 . 3. the aggregate itself may contract an appreciable amount upon drying. limestone. some structural grade lightweight aggregates. TABLE 3.3 3.0060 with an average for the 182 cements tested of about 0. Tests by Lerch1.Coarse and fine aggregates. will have greater or less shrinkage than a cement meeting requirements for some other type of cement. However. the amount of shrinkage observed on a small laboratory specimen will be greater than the shrinkage of the concrete in the structure.3 show that for short curing periods Type II cement pastes exhibited considerably less shrinkage than Type I pastes. Skalny. lower Na2O and K2O contents.0030. compressibility of the aggregate has the greatest influence on the magnitude of drying shrinkage of concrete. 2. 2.4. The shrinkage of a con- 400’ 2. The surface area of aggregate. (19 mm) to 11/2 in. 3. Concrete proportioned for pump placement with excessively high sand contents will exhibit significantly greater shrinkage than will similar mixes with normal sand contents. (75 to 100 mm) slump concrete. slump (25 to 50 mm).050 I . The large effect that the maximum size of aggregate has on the water requirement of concrete is shown in Fig. Bureau of Reclamation 3. clays.1). This will result in a lower water content per unit volume of concrete and thus lower shrinkage.S.060 u % 0. MAXIMUM SIZE OF AGGREGATE Fig. 3. is shown in Fig. 3. the water requirement of a concrete made with 3/4 in. From the above discussion it must be concluded that. in order to obtain desired workability. will substantially increase shrinkage and thus cracking of the concrete. Tests reported by Tremper and Spellman3. The large increase in shrinkage with increase in water content was demonstrated in tests made by the U. Any practice that increases the water requirement.224R-12 ACI COMMITTEE REPORT + 119 142 166 190 kg/m3 crete can be minimized by keeping the water con- 5 0. Also shown in Fig. and thus prevent the use of the maximum amount of coarse aggregate resulting in increased shrinkage. which must be coated by cement paste. but only 310 lb/yd3 (184 kg/m31 for a 1 to 2 in.5 (237) 19. Their data show that as the cement factor was increased from 470 to 752 lb/yd 3 (279 to 446 kg/m3) the water content remained nearly constant. This substantial reduction in water content would significantly reduce the drying shrinkage.Typical effect of water content of concrete on drying shrinkage (Reference 3.7 Maximum size of aggregate has a significant effect on drying shrinkage. This effect of temperature on water requirement as given by the U. For example.020 z z is 0. The amount of mixing water required for concrete of a given slump is greatly dependent on the maximum size of aggregate.3 Effect of water content and mix proportions The water content of a concrete mix is another very important factor influencing drying shrinkage. Bureau of Reclamation. This substantial reduction in water content would result in a lower drying shrinkage. for example. . This 40 lb (24 kg) reduction in water content would reduce the 1 year drying shrinkage by about 15 percent. produced concretes exhibiting low shrinkage characteristics.4. 3. (38 mm) decreases the water requirement from 340 to 300 lb/yd3 (202 to 178 kg/m3). 3. (19 mm) size aggregate is 340 lb/yd3 (202 kg/m3) for a 3 to 4 in. The total volume of coarse aggregate is a significant factor in drying shrinkage. An increase in water content also reduces the volume of restraining aggregate and thus results in higher shrinkage.2 . For example. such as the use of high slumps.3. while percentage of fine aggregate was reduced.010 200 240 280 320 Ib/yd3 WATER CONTENT OF CONCRETE Fig. The use of a poorly graded fine or coarse aggregate may result in an oversanded mix. 3.3 is the effect of slump on water requirement.Effect of aggregate size on water requirement of non-air-entrained concrete (ACI 211.8 A typical relationship between water content and’drying shrinkage is shown in Fig. slump. is the temperature of the fresh concrete." 0.1 shows.4 show that the cement factor has little effect on shrinkage of concrete. it would permit a reduction of the water content by 33 Ib/yd 3 (20 kg/m 3) and still maintain the same slump. The data plotted in this figure.S.5 75 150 m m 350 (208) 300 (178) 250 (148) (119) 200 3/8 3/4 1 1/2 3 6 in. increasing the aggregate size from 3/4 in.3. to minimize the drying shrinkage of concrete. high temperatures of the fresh concrete or the use of smaller size coarse aggregate. Another important factor which influences the water requirement of a concrete. but it is more effective in resisting the shrinkage of the cement paste.0 37. and thus its shrinkage. taken from ACI 211. if the temperature of fresh concrete were reduced from 100 to 50 F (38 to 10 C).3 . tent of the paste as low as possible and the total aggregate content of the concrete as high as possible. decreases with increase in size of aggregate. Not only does a large aggregate size permit a lower water content of the concrete. that for a 3 to 4 in.3. Aggregate gradation also has some effect on shrinkage. and slates which have high absorptions. 3.8). the water content of a mix should be kept to a minimum. 7 32. as noted in Section 3. Tests made by the California Department of Transportation 3. the lower the shrinkage. it was observed that the use of some of these pozzolans increased drying shrinkage although they had little effect on the water content of the concrete.7 Influence of size of member .4.The size of a concrete member will influence the rate at which moisture moves from the concrete and thus influence the rate of shrinkage. tuffs and pumicites are pozzolans used in portland cement concrete. This is substantiated by the test results of the California Department of Transportation3. However. Although the strength increases with age. will reduce drying shrinkage (AC1 517). DEPTH BELOW CONCRETE SURFACE Fig. the increase in the amount of air voids would increase drying shrinkage. prolonged moist curing may not necessarily be beneficial. will result in a substantial increase in drying shrinkage. since the pre’cast members are unrestrained. However. Steam curing at atmospheric pressure. TEMPERATURE OF FRESH CONCRETE Fig.Effect of temperature of fresh concrete on its water requirement (Reference 3. All of these observations are based on results of tests made on laboratory size specimens. However. Those most commonly used include air-entraining admixtures. 14. which is commonly used in the manufacture of precast structural elements. 3. it will usually not result in a decrease in drying shrinkage. 3. 3. diatomaceous earth. Also.6.5 Effect of pozzolans . 3. set-retarding admixtures. The use of calcium chloride.9). despite clearly greater shrinkage of the concretes with pozzolans in laboratory tests on small size specimens. it will reduce its tendency to crack.3. especially at the early ages of drying.44showed that the 7 day shrinkage of a concrete containing 1.4.8 Some air-entraining agents are strong retarders and contain accelerators which may increase drying shrinkage by 5 to 10 percent. the shrinkage of the concrete containing calcium chloride was only about 40 percent greater than that of the control mix.Rates of drying of concrete exposed to 50 percent relative humidity (Reference 3.1 26.5 . 3. The use of some natural pozzolans can increase the water de- mand as well as the drying shrinkage of the concrete.CONTROL OF CRACilNG 224R-13 4. As far as the cracking tendency of the concrete is concerned. with and without pozzolan. 3.2 378 OC 310 084) 300 (I 78) 290 (I 72) 280 (166) 0 270 (160) 0 260 (154140 a a 50 60 70 80 90 100 OF w iii -0 w 4 8 I2 16 20 24 28 in.4 Effect of chemical admixtures . a common accelerator.6 Effect of duration of moist curing . This may explain the negligible difference in shrinkage cracking of field structures.4 10.8).4. because stream curing will produce a high early-age strength of the concrete. while others may increase the shrinkage of the concrete. although the later age shrinkage of these concretes will be about the same as that of corresponding mixes with no admixtures.4 . Although the use of water-reducing and set-retarding admixtures will permit a reduction in the water content of a concrete mix. because entrainment of air permits a reduction in water content with no reduction in slump. and 28 days before drying was started.Car1son3. and accelerators.7 and Fig.Fly ash and a number of natural materials such as opaline cherts and shales. after 28 days of drying.Chemical admixtures are used to impart certain desirable properties to the concrete.6 reported that the duration of moist curing of concrete does not have much effect on drying shrinkage.0 15.6 21.4. 3. the modulus of elasticity also increases by almost as large a percentage. Some fly ashes have little effect on drying shrinkage.’ which show substantially the same shrinkage in concrete that was moist cured for 7. Also. water-reducing admixtures. Carlson3*’ has shown .4. and the net result is only a slight increase in the tensile strain which the concrete can withstand. the larger the concrete member.0 percent of calcium chloride was about double that obtained for the control mix without admixture. It would be expected that when using an air-entraining admixture. Actually some of these admixtures may even increase the shrinkage at early ages of drying. the shrinkage is not appreciably affected by air contents up to about 5 percent. 5 x 7. therefore. A lower water content can be achieved by using a well-graded aggregate.5x7. using adequate and properly positioned reinforcement. A larger aggregate size allows an increase in aggregate volume and a reduction in the total water required to obtain a given slump.5 cm) was more than 50 percent greater than that of the concrete prism having a cross section of 5 x 6 in.224R-14 ACI COMMITTEE REPORT that for a concrete exposed to a relative humidity of 50 percent. and using control joints. The CEB-FIP Code give quantitative recommendations on the control of cracking due to shrinkage. and the creep or relaxation of the concrete. Control of cracking by correct construction practices is covered in Chapter 8 of this report. Hansen and Mattock3. the cracking tendency is due not only to the amount of shrinkage. It will be noted that the shrinkage of the prisms having a cross section of 3 x 3 in. which includes specifications to minimize drying shrinkage (Section 8. however. listing various coefficients to determine the shrinkage levels that can be expected. The control of cracking consists of reducing the cracking tendency to a minimum. 3. They found that both the rate and the final values of shrinkage and creep decrease as the member becomes larger. Fig. such internal cracking is not necessarily harmful. giving the results of shrinkage tests obtained on four different size concrete prisms. 3.As mentioned previously. (75 mm) in 1 month and about 2 ft (0. Cracking can also be minimized by the use of expansive cements to produce shrinkage-compensating concretes. thus offering little or no help to the cracking tendency. many surface coatings such as allpurpose paints are ineffective. Test results of several studies carried out to compare the shrinkage of concrete in walls and slabs in the field with the shrinkage of small laboratory specimens have shown. but there are probably many other materials which will slow the evaporation enough to be beneficial.6 .4. 3. stiffer consistency. A third way to reduce the cracking tendency is to apply a surface coating to the concrete. the reduction of water content by the use of water-reducing admixtures will not usually reduce shrinkage. The larger aggregate also tends to restrain the concrete more. the modulus of elasticity. 3. (7. and lower initial temperature of the concrete.5. the possibility of cracking must be expected unless the ambient relative humidity is kept at 100 percent or the concrete surfaces are sealed to prevent loss of moisture. that the shrinkage of the concrete in a field structure is only a fraction of that obtained on the laboratory specimens.5 shows his theoretical curves for the drying of slabs. (12. However. Some factors which reduce the shrinkage at the same time decrease the creep or relaxation and increase the modulus of elasticity. Shrinkage-compensating concretes are discussed in Section 3. Chlorinated rubber and waxy or resinous materials are effective coatings.5 .Effect of specimen size on drying shrinkage of concrete (Principal author’s data). Drying shrinkage can be reduced by using less water in the mix and larger aggregate size. Since various kinds of restraint prevent the con7. This means of controlling cracking has not been used to its full potential and should be given better consideration. As an example of the effect of specimen size on shrinkage is the data presented in Fig. Even in laboratory tests the size of the specimen used has a significant influence on shrinkage. This significant effect of size of member on drying shrinkage of concrete must be considered when evaluating the potential shrinkage of concrete in structures based on the shrinkage of concrete specimens in the laboratory. 3.1 Reduction of cracking tendency . As discussed in Section 3. Emphasis should be placed. because they permit the moisture to escape almost as fast as it reaches the surface. which will prevent the rapid loss of moisture from within.5 10 x 10 10x12 5 12.Control of shrinkage cracking Concrete tends to shrink due to drying whenever its surfaces are exposed to air of low relative humidity. and although this may result in internal microcracking.6 m) in 10 years.5x 15 cm crete from contracting freely.6. Any slowing of . Any measure that can be taken to reduce the shrinkage of the concrete will also reduce the cracking tendency. but also to the degree of restraint.4.10 made an extensive investigation of the influence of size and shape of member on the shrinkage and creep of concrete. The rate and magnitude of shrinkage of a small laboratory specimen will be much greater than that of the concrete in the structures. as expected.5 x 15 cm).6). 3x3 I I I 4x4 4x5 I 5x6 in AVERAGE END AREA DIMENSION OF CONCRETE PRISM ( LOG SCALE ) Fig. Another way to reduce the cracking tendency is to use a larger aggregate size. on modifying those factors which produce a net reduction in the cracking tendency.6. drying will penetrate only about 3 in. These grooves should be sealed on the outside of the wall to prevent penetration of moisture. The properties and use of expansive cement concretes is published in numerous papers and reports.2 R e i n f o r c e m e n t . Although the use of such reinforcement to control cracking in a relatively thin concrete section is practical.16Ym PORTLAND CEMENT CONCRETE . This basic concept of the use of expansive cement to produce a shrinkage-compensating concrete is illustrated in Fig. such as walls.CONTROL OF CRACKING 224R-15 the rate of shrinkage will be beneficial. slabs and floors.Shrinkage-compensating concretes Shrinkage-compensating concretes made with expansive cements can be used to minimize or eliminate shrinkage cracking. it will make its own “joints” by cracking. the Type K shrinkage-compensating expansive cement is most commonly used in the United States.COMPENSATING CONCRETE. concrete may be able to withstand two or three times as much slowly applied shrinkage as it can rapid shrinkage. slabs or pavements. used in adequate amounts. 3.6 . Thus. By distributing the shrinkage strains along the reinforcement through bond stresses. The initial precompression of the STEEL\ _---_B--ORIGINAL LENGTH T A b T ___++IC~*___ t EXPANSION PUTS STEEL IN TENSION AND CONCRETE IN COMPRESSION M STRESS LOSS DUE TO SHRINKAGE AND CREEP RESIDUAL EXPANSION OR. is not provided with adequate joints to accommodate shrinkage. roof slabs. concrete minimizes the magnitude of any tensile stress that may ultimately develop due to shrinkage. If a sizable length or expanse of concrete.* 3. In a reinforced concrete. Contraction joints in walls are made.8. CURING DRYING p- SHRINKAGE. A typical length change history of a shrinkagecompensating concrete is compared to that of a portland cement concrete in Fig. it is not needed in massive structures such as dams due to the low drying shrinkage of these mass concrete structures. DAYS Fig. The minimum amount and spacing of reinforcement to be used in floors. p = 0. relieving the stress in the wall and thus preventing formation of unsightly cracks. Cracking of the wall due to shrinkage should occur at the grooves. 3.7 MPal. The level of compressive stresses developed in the shrinkage-compensating concretes ranges from 25 to 100 psi (0.5.Length change characteristics of shrinkage-compensating and portland cement concretes (Relative humidity = 50 percent). 0 I 50 I 100 I 150 I 2oc AGE OF CONCRETE.3-11* 3*12 Of the several types of expansive cements produced. l r/ . by fastening to the forms wood or rubber strips which leave narrow vertical grooves in the concrete on the inside and outside of the wall.8 . 7 . because concrete has a remarkable quality of relaxing under sustained stress. 3.The use of joints is the most effective method of preventing formation of unsightly cracking.3 Joints . the contraction of the concrete will result in a reduction or elimination of its precompression. respectively. -+j SMALL CONTRACTION .Basic concept of shrinkage-compensating concretes. Qr 3. for example.6.2 to 0. and walls is given in AC1 318.P r o p e r l y p l a c e d r e inforcement. will not only reduce the amount of cracking but prevent unsightly cracking. Each job must be studied individually to determine where joints should be placed. 3. and thus reduce or eliminate the tendency to cracking. Joint location depends on the particulars of placement.7. the expansion of the cement paste during the first few days of curing will develop a low level of prestress inducing compressive stresses in the concrete and tensile stresses in the steel. . the cracks are distributed in such a way that a larger number of very fine cracks will occur instead of a few wide cracks. The amount of reinforcing steel normally used in reinforced concrete *Guidance on joint sealants and control joint location in slabs is available in ACI 504 and in ACI 302. Sawed joints are commonly used in pavements. . When subjected to drying shrinkage. 3. ACI C o m m i t t e e 2 2 3 . SP-38. 33. and crack control in structures reinforced with fiber glass rods is not addressed in this report. Bailey. 3. L. 1964.” Proceedings. ASTM. New York State Department of Transportation. V. curing by sprayed-on membranes or moisture-proof covers have been successfully utilized. 3.4. Proceedings V. 1973. ASTM. “Influence of Size and Shape of Member on the Shrinkage and Creep of Concrete. Crack control is important to promote the aesthetic appearance of structures. pp.. Hansen. 3. and higher allowable stresses in prestressed concrete design. Donald L. 1946. S.”4.. Several of the most important crack prediction equations are reviewed in the previous committee report. and for many structures. 3 .Comparison of Laboratory and Field Performance. Internal cracking in concrete can start at stress levels as low as 3000 psi (20. Inadequate curing of shrinkage-compensating concrete may result in an insufficient expansion to elongate the steel and thus subsequent cracking during drying shrinkage.1 . Recently. Concrete Manual. Lerch. M o s t o f t h e m a r e r e p o r t e d i n ACI Bibliography No.” ACI J O U R N A L . 327-336. For slabs on well saturated subgrades. 419-437. 3. Arni. 69-8. 3. 63. Verbeck. To take full advantage of the expansive potential of shrinkage-compensating concrete in minimizing or preventing shrinkage cracking of unformed concrete surfaces. shrinkage. 491 pp. Proceedings V. R. No. H.C. 1938. This chapter is concerned primarily with cracks caused by flexural and tensile stresses. experience is limited. but temperature. Alan H. No. Proceedings V. 1975. Blaine.4. Carlson. 3. Tremper. 267-290. 1958. “Carbonation of Hydrated Portland Cement. Carlson.. 1969. 30 pp. W.6. Nawy and Peter Gergely. “Shrinkage of Concrete .224R-16 ACI COMMITTEE REPORT made with portland cements is usually more than adequate to provide the elastic restraint needed for shrinkage-compensating concrete. D. Reference 4. It is expected. N.4. 583-610.5... pp.-Feb. 46.. most of them are reviewed in the previous committee report 1. Engineering Research and Development Bureau. 1969. 3. Specific recommendations and information on the use of shrinkage-compensating concrete are contained in ACI 223.Control of cracking in flexural members* Hydration and Properties of Portland Cement Pastes. however. pp. shear and torsion may also lead to cracking.” Proceedings. Albany. U. Aug.” Building Science Series No.9. T.. D. 67. Skalny.” ACI JOURNAL. 38. “Drying Shrinkage of Concrete as Affected by Many Factors. bins and silos. STP-205.1. 3. 30-61.. “ E x p a n s i v e C e m e n t Concretes-Present State of Knowledge. References 3. the control of cracking may be as important as the control of deflection in flexural members. and Spellman. Jan.3. it is important that positive and uninterrupted water curing (wet covering or ponding) be started immediately after final finishing. 3. such as reinforced concrete tanks. Highway Research Board. V. “The Influence of Gypsum on the Chapter 4 . research has shown that isolated cracks in beams in excess of twice the width of the computed maximum can *Principal authors: Edward G. 9 on crack control. Klein Symposium on Expansive Cement Concretes. that future committee documents will address crack control in structures using this and other new systems as they come into use. Brunauer. National Bureau of Standards. Washington. H. 77 pp.. Nov. ” M o n o g r a p h 74.” Highway Research Record. “Drying Shrinkage of Large Concrete Members.12.” ACI JOURNAL . Denver. Mar. 12 pp. 15. “Hardened Cement Pastes of Low Porosity: Dimensional Changes. T. Torben C. American Society for Testing and Materials. 2. crack control plays an important role in the control of corrosion by limiting the possibilities for entry of moisture and salts which.2. Roy W.7. pp.8.3 Others are referenced in this chapter. and Mattock. is not covered in this report. Bureau of Reclamation. which usually means that about 90 percent of the crack widths in the member are below the calculated value. 1 1 . 8. Most equations predict the probable maximum crack width.” Cement and Concrete. see Reference 4. For information on cracking concrete in these structures. pp. 1.Introduction With the regular use of high strength reinforcing steel and the strength design approach for reinforced concrete. D.1 Cracking in certain specialized structures. 4. 17-36.. William.2 and ACI 313. Roy W. 627 pp. together with oxygen. Reichard. 1970..Crack control equations for reinforced concrete beams A number of equations have been proposed for the prediction of crack widths in flexural members. Washington.Shrinkage of Hardened Portland Cement Pastes and Concrete.4To date. and Evans. “Interrelations Between Cement and Concrete Properties: Part 4 . No.7 MPa) in the reinforcement. No.1Pand in key publications listed in the references.1’ Additional work presented in the CEB-FIP Model Code for Concrete Structure gives the European approach to crack width evaluation and permissible crack widths. can set the stage for corrosion. . 4. Philadelphia. 1963.. fiber glass rods have been used as a reinforcing material.C. 3..2 . However. 3. 1966. 8th Edition. “Creep and Drying Shrinkage of Lightweight and Normal Weight Concrete.” Research Report No. 1937. 1252-1297. pp. 3. American Concrete Institute.. George J. Part II.: and Yudenfreund. pp. Feb. Extensive research studies on the cracking behavior of beams have been conducted over the last 5 0 y e a r s . Detroit. National Bureau of Standards.1 contains an extensive review of cracking in reinforced concrete structures. J.S.10. The steel stress is the most important variable.2 p L.2..2) cracking at the tensile strength of concrete influence of load repetitions and load duration . fs steel stress at the crack f II = steel stress at the crack due to forces causing K = bond coefficient.25 to 1. = (4. The bar diameter is not a major variable.*-4 though generally the coefficient of variation of crack width is about 40 percent. Based on the analysis. in. 4. in.la) (4. (19 mm) cover and reinforced with steel of 60 ksi (414 MPa) or lower yield strength results in large reinforcement spacings. The cracking behavior in thick one-way slabs is similar to that in shallow beams.5) x 1 0 . (4.091 rt.35 is used.. While application of Eq. ksi fJ = area of concrete symmetric with reinforcing A steel divided by number of bars. which may be excessive based on Table 4.1. 4.0 for ribbed bars. = reinforcing steel stress. Ed. 4.016 in. . the provisions of Code Section 7.2) can be adequately applied if p = 1.013 in. (4. its application to one-way slabs with standard 3/4 in. A .4 mm).3) is valid in any system of measurement.lb) w.6 uses Eq.076~fs ~AX D3 (4.4-1 Evidence also exists indicating that this range in crack width randomness may increase with the size of the member? Besides limiting the computed maximum crack width to a given value. 2. but not the only geometric consideration. 1. (f.4) where (4.2 CEB recommendations . (25. The thickness of the concrete cover is an important variable. reflecting distance from neutral axis to the reinforcing steel.2 tb = bottom cover to center of bar.l& (4. . in. = most probable maximum crack width at level of reinforcement. = side cover to center of bar. = When the strain. AC1 340. t.la) yielded the following equation w = 0.33 mm). in. The area of concrete surrounding each reinforcing bar is also an important geometric variable. The equations that were considered to best p r e diet the most probable maximum bottom and side crack widths are: W* = where W = most probable maximum crack width.Crack control recommendations proposed in the European Model Code for Concrete Structures apply to prestressed as well as reinforced concrete and can be summarized as follows: The mean crack width. in.5) and represents the average strain in the steel. V-JX E.2a) 0 . (4.21 ACI Committee 224 recommendations . d c thickness of cover from tension fiber to center of bar closest thereto. Eq. However. (4. w. for exterior exposure based on a crack width value of 0.41 mm).Requirements for crack control in beams and thick oneway slabs in the ACI Building Code (ACI 318) are based on the statistical analysis4-6 of maximum crack width data from a number of sources.2) with p = 1. in. The Code allows a value of z = 145 kips per in.2) becomes w = 2. in. for interior exposure corresponds to a limiting crack width of 0.5 indirectly limit the spacing of such reinforcement in one-way slabs. 0 9 1 v-a p (f. Crack control equations recommended by ACI Committee 224 and the Comite Euro-International du Beton (CEB) are presented below. 5. wm in beams is expressed in terms of the mean crack spacing.1R contains design aids for the application of Eq.3) strain in the reinforcement Eq. srm such that Kn = L&n where W* = most probable maximum crack width at bottom of beam. in. = ratio of distance between neutral axis and P tension face to distance between neutral axis and centroid of reinforcing steel = 1. the designer should estimate the percentage of cracks above this value which can be tolerated. AC1 318 Section 10. (0. The size of the bottom crack width is influenced by the amount of strain gradient from the level of the steel to the tension face of the beam. in the steel reinforcement is used instead of stress. the following general conclusions were reached: 1.3 -1-G.cQi- (4. (0.CONTROL OF CRACKING 224R-17 sometimes occur.20 in beams h1 = (4. f. = l Using the specified cover in AC1 318. maximium allowable z = 175 kips per in. (4.2a). Eq. (10. For one-way slabs having a clear concrete cover in excess of 1 in.5) x 10-3 0. 3. Simplification of Eq.2 in the following form 2 = f. t.4) of AC1 318-771 to beams gives adequate crack control values. (4..6. 8 and 340.4 .25 (chosen to simplify calculations though varies between 1.-A.05 -! E QR S = = = 1" = Qrl (4. or 40 percent of the design yield strength fy.8) where the radical rl = db.224~018 ACI COMMITTEE REPORT The mean crack spacing is S rm (4. It is important to note that these values of crack width are w &. (19 mm) for interior exposure].9. presumably equivalent to the probable maximum value.Tolerable crack widths versus exposure conditions in reinforced concrete Table 4. in.1) 1..5.7~~.Crack control in two-way slabs and plates Crack control equations for beams underestimate the crack widths developed in two-way slabs and plates4. in the equation. For simply supported slabs. is given as 1. 0. a value of k = 2. In addition. References 4. is termed the grid index. whereas it is a major variable in the crack control equations for beams. use s2 = 12 in.of V steel .7) = = f d action square slabs and plates. For zones of flat plates where transverse steel is not used or when its spacing s2 exceeds 12 in. depending on arrangement of bars and type of external forces. 4.. A t = effective area in tension.ft -width _ . the clear concrete cover in two-way slabs and plates is nearly constant [3/4 in.9) where values of the kl = 29 x 100~ times the k values previously listed. The cracking mechanism in twoway slabs and plates is controlled primarily by the steel stress level and the spacing of the reinforcement in the two perpendicular directions. If strain is used instead of stress.4 for ribbed bars x3 = depends on the shape of the stress diagram.12 (dbt + 2CJ A characteristic value of the crack width. = 0.3c + 0.6 x 1O-s (as defined in Section 4. Analysis of data in the only major work on cracking in two-way slabs and plates4s7 has provided the following equation for predicting the maximum crack width: w= where Cl is clear concrete cover measured from the tensile face of concrete to the nearest edge of the reinforcing bar in direction b& 1VW w = crack width at face of concrete. and can be transformed into ] k = fracture coefficient.8) becomes (4.75 but larger than 0. in. Eq. in.7 _“. /A. x2 = 0. k = 1. P .5. in. or when the ratio of short to long span is less than 0. in the case of slabs. The table is based primarily on Reference 4. Interpolated k values apply for partial restraint at the boundaries.20 and 1. spacing of the reinforcement in perpendicular direction “2”. this is the direction for which crack control check is to be made active steel ratio Area per .1 is a general guide for tolerable crack widths at the tensile face of reinforced concrete structures for typical conditions and is presented as an aid to be used during the design process.6) where P fs d b1 s1 s2 46 = = c = clear concrete cover S bar spacing. For span aspect ratios 0. not more than halfway to the neutral axis A simplified formula canbe derived for the mean crack width in beams with ribbed bars. (4.3 . caused by flexural load Subscripts 1 and 2 pertain to the directions of reinforcement.125 for bending QR = A. spacing of the reinforcement in direction “l”.1 x 1O-5 is applicable.1R contain design aids for the application of these recommendations. 4.8 x lO-5 for uniformly loaded restrained two-way . the value of k should be multiplied by 1. For concentrated loads or reactions. it is limited by a line c + 7d.2.35) actual average service load stress level.s21et. direction of reinforcement closest to the outer concrete fibers.5. limited to 15d. having a value k = 2.7 and do not tell the designer how to space the reinforcement. ksi diameter of the reinforcement in direction “1” closest to the concrete outer fibers. w.sI: n s (4. from the tension face for beams. The determination of the decompression moment and.One approach to crack prediction. 4. and the use of crack width limits should depend on the fluctuation and magnitude of the live load.5.15) (0. has two steps.11-4.5.1 Crack prediction equations . However.23 than indicated by these analyses.006 0. Several other equations have been proposed.Tolerable crack widths. may sometimes b e preferable for corrosion control in certain environments. Furt h e r m o r e cracks close upon removal of the load. 0. a larger cover.08 mm)] and therefore. and the lack of sufficient data. (0.“0 Limited evidence seems to indicate that unbonded members develop larger cracks than bonded members.5 .10) not always a reliable indication of the corrosion and deterioration to be expected. Then the member is treated as a reinforced concrete member and the increase in stress in the steel is calculated for the additional loading.CONTROL OF CRACKING 224R-19 TABLE 4. Transverse reinforcement (stirrups) must be designed to restrict these cracks. The CEB Model Code has the same equation for the prediction of the crack width in prestressed members as in nonprestressed members (see Section 4.2. First the decompression moment is calculated. soil Deicing chemicals Seawater and seawater spray: wetting and drying Water retaining structures* *Excluding nonpressure pipes Tolerable crack width.“’ indicate that there is no general relationship between cracking and corrosion in most circumstances. and the consequences of under-reinforcement . the number of variables is greater in prestressed members. but away from the end face. Cracks form in these members when the tensile stress exceeds the modulus of rupture of the concrete (Sfl to 90 under short-term conditions). w h i c h r e l a t e s i t t o t h e nonprestressed case.03 to 0.4. In particular. is small [about 0. A multiplication factor of about 1.1 and 4. Two types of cracks may develop: spalling cracks which begin at the end face (loaded surface) and propagate parallel to the prestressing force.18) (0.30) (0.3 to limit crack width. the stress in the steel is complicated and unreliable unless elaborate methods are used.4. The increase in steel strain is calculated from the decompression stage. especially.6 .003 in.2 Allowable crack widths . after removal of the major portion of the live load. 4. in.2. The expressions given for crack prediction in nonprestressed beams may be used to estimate the cracks for the load increase above the decompression moment.012 0.Some authors state that corrosion is a greater problem in prestressed concrete members because of the smaller area of steel used.10 For this reason.41) (0.2. Classical and finite-element analyses show similar stress distributions for which the stirrups are to be provided. rather than deformed bars. 4. since experimental evidence shows that higher stresses can result. The control of these cracks is necessary mainly for esthetic reasons.004 (mm) (0. even if it leads to a larger surface crack width. These are not much less accurate than the more complicated methods.001 in. 4.22T 4. approximate methods for crack width prediction are attractive. moist air. Thus. at the same time. are used nearest to the beam surface in the prestressed member to account for the differences in bond properties. are used extensively. crack control is usually not necessary if the live load is transitory. recent research results4.4.2).007 0. in which cracks may appear under working loads. Nonprestressed deformed bars may be used to reduce the width of the cracks to acceptable levels. precludes further refinements at this date. The difficulty with this approach is the complexity of calculations. However. the designer must exercise engineering judgment on the extent of crack control to be used. The available experimental data are limited and. at which the stress at the tension face is zero. For many years stirrups were designed to take the entire calculated tensile force based on the analysis of the uncracked section. When used in conjunction with the recommendations presented in Sections 4. The residual crack width.016 0. The cracks in bonded post-tensioned members are not much different from cracks in pretensioned beams. covering large variations in the variables.Flexural cracking in prestressed concrete Partially prestressed members. to 0.Anchorage zone cracking in prestressed concrete Longitudinal cracks frequently occur in the anchorage zones of prestressed concrete members due to transverse tensile stresses set up by the concentrated forces. reinforced concrete Exposure condition Dry air or protective membrane Humidity. it should be expected that a portion of the cracks in the structure will exceed these values by a significant amount.23 Such cracks may lead to (or in certain cases are equivalent to) the failure of the member. The prediction of crack widths in prestressed concrete members has received far less attention than in reinforced concrete members.4. and bursting cracks which develop along the line of the force or forces.1 .5 is needed when strands. Atomic Energy Commission. 1956.. Fifth IABSE Congress (Lisbon. 1969. 217-219. and Neuwerth. Fritz. Apr. pp. “Crack Control Through Reinforcement Distribution in Two-Way Acting Slabs and Plates.22.” Concrete (London). pp. 4. and Loov.19. Sten. Plates and Beams with Fiber Glass as Main Reinforcement.16. 1975. “Crack Spacing and Crack Widths Due to Normal Force and Bending Moment. 1958. May-June 1977.” ACI JO U R N A L . 4.” 4. Precast blocks with helical reinforcement may be used when the prestressing forces are large. Kenneth W. pp. C. and Rowe. 4. Edward G. W. No.” ORNL-TM-2378. No. 4. SP-30. 11. w = O.S. Zielinski..18. designs have been based on cracked section analyses. “Corrosion of Reinforcing Steel in Concrete and Its Relation to Cracking. 179-185..” Journal. 1969. No. 1977. S. Rao. pp.224R-20 ACI COMMITTEE REPORT can be serious. ACI Committee 224. Detroit. V. it is advisable to provide more steel than required by this type o f analysis. 4. Anchorage Zone Problems. 26 pp . 3. Oak Ridge National Laboratory. pp. 63.. and Ahamed Kurty. “Relative Merits of Plain and Deformed Wires in Prestressed Concrete Beams Under Static and Repeated Loading. 1968.. No. 4. 12. Raju N.. 65. it is important to place the first stirrup near the end surface..1. 4.lOf..2 pp. L. 1977. 52.4. 4. Nawy. “Crack Control in Reinforced Concrete Structures. pp. C. Partially Prestressed. W. International Association for Bridge and Structural Engineering. New York. Detroit. Zurich. and Control of Cracking in Concrete.. and to distribute the stirrups over a distance equal to at least the depth of the member to fully account for both spalling and bursting stresses. Zurich.” Proceedings.11. Concrete Tanks. V.” The Structural Engineer (London).. R.” ACI JOURNAL . Tenn. 4. pp. 4... 333-346. 1968. 56. Krishna. 4. “Cracking in Reinforced Concrete Flexural Members. pp. 1401-1410. Bennett. E.7. Bennett.” IABSE Surveys No. E. International Association for Bridge and Structural Engineering. 4. and Chandrasekhar. “Anchorage Systems in Prestressed Concrete Pressure Vessels. 1978.2. Data are limited but it appears that the maximum tensile crack width may be expressed approximately in a form similar to that used for flexural crack width. E. G. and Dave. Nawy. 35-11. Detroit.&tA x 10-3 (4. “Tests on Prestressed Reinforced Concrete Beams. 1956).” Proceedings.. 77-81... May 1976. 526 pp. Bate.. Institution of Engineers (Calcutta).14. 87-117. 851-862. E. Aug. Oct. Feb. U.25 Spalling cracks form between anchorages and propagate parallel to the prestressing forces and may cause gradual failure. V. Prestressed Concrete Institute.Tension cracking The cracking behavior of reinforced concrete members in tension is similar to that of flexural members.. 1966.. 457-462. V. Gergely.. Stevens. 56A. ST2. More recently.15. pp. S.” Publication No. Proceedings. Arthur P. Nawy.4.. C.” Building Science. “Maximum Crack Width in Reinforced Concrete Flexural Members. 3. 4.” ACI JOURNAL .5. E. V.. 4. 103.” Proceedings. Institution of Civil Engineers (London). “An .6. “Crack Control in Concrete Structures. G. S. p.10) References 4.4. Gergely. 9. F. except that the maximum crack width is larger than that predicted by the expressions for flexural members. July 1971. V. No. Leonhardt. Arthur H. American Concrete Institute. Edward G. 69. Proceedings V. K. A.27 The lack of strain gradient. and Blair. 825-836. 4. G. pp. U. and Ramaswamy. Hutton. Nawy. Nawy. 8.” ACI JO U R N A L. Mar. “Minimum Wall Thickness of Circular Causes. 1956. an empirical equation has proven to be quite usefu1. Mechanism. 57 pp.21. 1972. V. A design procedure for post-tensioned members using a cracked section analysis4. 10. 30-47. Proceedings V. “Flexural Behavior of Prestressed. “Causes.17. John Wiley and Sons. Ake. 4. 49. 2. No.” Document D2:1970. International Association for Bridge & Structural Engineering. American Concrete Institute.P. 4. and Huang. Deflection. pp. especially when the force acts near and parallel to a free edge. 473-502. N.. Mechanism. 1978. G.” Proceedings. Edward G. 1-41. T. Holmberg.. Edward G. V. Design of Prestressed Concrete. 4. is probably the reason for the larger tensile crack width. Oak Ridge. pp.. Nilson. 1969. Stockholm.” ACI Bibliography No. S. 237. 487-496. Stephen C. 47. “Crack and Deflection Control of Pretensioned Prestressed Beams.23. and Control of Cracking in Concrete. 10. Proceedings V. 4.3. V. W. and Lutz. 4. Dec. Dec. a n d resultant restraint imposed by the compression zone of flexural members. No.12.10. 1971. V. “Further Studies on Flexural Crack Control in Structural Slab Systems. “Flexural Tests on Beams Prestressed to Different Degrees of Prestress. A.. pp. 22. Since analyses show that the spalling stresses in an uncracked member are confined to near the end face. 1970. Gandotra. and Ultimate Load of Concrete Slab Systems. Peter. Zurich. Beeby. American Concrete Institute. Abeles. “Test Performances and Design of Concrete Beams with Limited Prestress. R. 707-720. and Lindgren.” The Structural Engineer (London). Yerlici. ASCE. Paul W. National Swedish Council for Building Research. 8. 12..13. S4/77.. 1971.. V. A. For pretensioned members.. “Calculation of the Width of Cracks in Class 3 Prestressed Beams. “Behavior of Concrete Slabs. No. Apr. “Flexural Behavior of Pretensioned Concrete Beams with Limited Prestress.24 has found acceptance with many designers.8. J.” Journal. SP-20. June 1973. 9. No.20.S. Peter. 4. 4. 4. pp.9.7 . 421-440.. Basavarajuiah. P. B. 3.” Cracking. l-49. R. Institution of Civil Engineers (London). pp. “Cracks in Prestressed Concrete Beams. E. Nov. Leroy A. pp. J. Clark. and Reinforced Concrete Beams.26T 4.. 5. 1967.7 Under initial loads.1-5. cracks in flexural members are triangular in shape increasing in width from the neutral axis to the soffit. of creep. For short-term static and fatigue loading. 5.25.9 indicate that a doubling of crack cycles width with time can be expected. 1 0 Cracks grow in width under sustained loading at a decreasing rate. crack width is a function of cover. 5. pp.. and Mattock. No.8 While there is a large scatter in the data.9 The rate of crack development. Gergely. the entire question of the importance of crack width on corrosion protection remains open. the adhesion bond between the steel and the concrete undergoes breakdown.8 An excep*Principal authors: David Darwin and Ernest K. 32 pp. Schrader. After about 2 years.7.26. Leroy A. The discussion in this chapter summarizes the major long-term factors which affect the crack control performance of concrete. V. The increase in crack width due to long-term or repetitive loading can vary between 10 percent and 1. especially in terms of corrosion of reinforcing steel and appearance. in which case the total number and width of cracks increase substantially after the loading has begun. Since additional cover also results in added surface crack width.5. the crack width at the reinforcement is approximately equal to the crack width at the surface. 1395-1410. however.5. V. For long term loading. 56-74. Therefore.5. V. The total amount of microcracking appears to be a function of the total strain and is largely independent of the method by which the strain is induced.4.Environmental effects Chapter 5 .” Journal.7. 1965. London.27. 1960.8. 4. 1962.10 As discussed in Chapter 4. 5 . Mar. pp.2. the spacing of cracks does not change with time at constant levels of stress. No. tion to this occurs at low loads or in beams with high percentages of reinforcement. Under most conditions.15. Nov. Proceedings. No. W. plus the strain induced in the concrete due to shrinkage..8 The largest percentage increase in crack width is then expected in flexural members subject to low levels of load. 1237-1256. long-term loading and repetitive loading seem to give about the same crack widths and spacing. the rate of growth is faster than the average observed surface strain at the level of the steel.5. and since this surface crack width appears to provide a good estimate of the crack width at the level of the steel.5. 4.” Technical Report No. crack width is proportional to the steel strain (including the effects of creep).CONTROL OF CRACKING 224-21 Investigation of the Stress Distribution in the Anchorage Zones of Post-Tensioned Concrete Members.7 At this stage. “Design of Anchorage Zone Reinforcement in Prestressed Concrete Beams.5. after a few years.24.5. No. Over a period of time. and Sozen.Introduction Cracking in concrete is affected by the long-term conditions to which the concrete element is subjected. 2. both sustained and cyclic loading increase the amount of microcracking in concrete.” ACI JOURNAL.14 Added cover is generally acknowledged as a method of improving the corrosion protection for reinforcing steel.” ACI JOURNAL . For both prestressed and reinforced concrete flexural members. 7 .-Apr. and are approximately uniform across the width of the beam. Oct.2 . M. Marshall. T.. and microcracks formed at service load levels do not seem to have a great affect on the strength or serviceability of concrete. 5. 12. A. 10..5. 63-75. The long-term effects of an adverse environment in both producing and in enlarging concrete cracks 5. long-term exposure and longterm loading extend the magnitude of cracks in both reinforced and plain concrete.8 and fatigue tests with up to one million 5.5.5. It does seem clear that crack widths predicted on the basis of short term static tests do not provide a precise guide to crack widths in structures actually in service. Microcracking due to long-term loading may well be an effect.11 and thus the width of surface cracks do not provide a good indication of the exposure of the reinforcing steel to corrosive conditions. Cement and Concrete Association. 11.Long-term effects on cracking* 5.3 . “Control of Horizontal Cracking in the Ends of Pretensioned Concrete Girders.5. rather than a major cause.12-5. however. 5. A. 5. Sept.. “Effects of Arrangement of Reinforcement on Crack Width and Spacing of Reinforced Concrete Members.Effects of long-term loading As discussed in Chapter 2. Aug.5.7-5. 5. H. P.5. however. 1965. information obtained from sustained loading tests of up to 2 . 4. since the cracks take more time to develop. However..” Journal.000 percent over the span of several years. Broms.5.8-5. Broms. 7. The effect of sustained or repetitive loading on macroscopic cracking. surface crack width is approximately proportional to the steel s t r a i n 5 . 9. 8 .5. pp. Prestressed Concrete Institute. Many questions remain as to the importance of crack width on the serviceability of reinforced and prestressed concrete members.-Oct.4. In most cases. “Crack Width and Crack Spacing in Reinforced Concrete Members. the width of a surface crack provides a good estimate of the crack width at the level of the reinforcing steel. Prestressed Concrete Institute. 5. Bengt B. 62.5. 5 . 4. 62. Bengt B.16 can be damaging to both concrete and . is considerably faster under repetitive loading.1 . and Lutz. cracks adjacent to reinforcement are restricted by the bond between the steel and the concrete. Proceedings V. pp. may be an important consideration in the serviceability of reinforced concrete members.4.. Also.3. alkaline solutions stored or used in the finished structure).. Proceedings. it will develop cracks when frozen. Paul W. Detrimental conditions may also result from the application of deicing salts to the surface of hardened concrete. “Increase in Crack Width in Reinforced Concrete Beams Under Sustained Loading. pp. 9. These materials are discussed in greater detail in Chapter 6.5 . curing water. (210 MPa) but did increase substantially (50 to 100 percent) over a 9 year period when the steel was 30 ksi (210 MPa) or more. AC1 201. If concrete is not resistant to freezing and thawing when critically saturated. and Gergely.: Brown. 64.5-22 Polymer impregnation is another method of introducing beneficial polymer systems into concrete. 1968. alkali-carbonate.5 to 7. admixtures or external sources (e. 1963.1. Stephen C. The potential for some expansive reactions.g. pp.. 1964). In more extreme cases. high tensile and compressive strengths and negligible permeability. This procedure creates a ‘layer’ of high quality material to the depth that has been impregnated.5*1gy 5. Nand K. Maximum crack widths did not increase with time when the steel stress was less than 30 ksi. 5.). Prestressed Concrete Institute.” Proceedings. If the aggregate used in the concrete is durable under freeze-thaw conditions and the.20p 5.18 give details on identification and evaluation of aggregate reactivity. 1968.. Sept. and should be made with a high-quality mix yielding low permeability. 5.4 . (38 mm) MSA concrete].7 MPa). strength of the concrete is high. deflection. The achievement of critical saturation in nonfrost-resistant concrete may be facilitated by the presence of preexisting cracks which allow entry of water more readily than would be the case otherwise. 1964. (19 mm) MSA concrete and 4. limitations on the alkali content of cement.224R-22 ACI COMMll-i=EE REPORT reinforcement. pp. Abeles. . C. crack width. Because of these desirable characteristics.Aggregate and other effects Concrete may crack as the result of expansive reactions between aggregate and alkalis derived from cement hydration. and Morrow. V. Referenees 5.. 916-922.21 Polymer-portland cement concretes have a large deformation capacity. the role of cracks as they effect the deficiencies in frost resistance will vary with the environmental conditions (e.g. “A Comparison Between Prestressed Concrete and Reinforced Concrete Beams Under Repeated Loading. Bate. V. Hence. it is not uncommon for cracks in the roadway deck of dams and navigation locks (caused either by thermal stress or shrinkage of the richer topping mix) to spall due to water which freezes in the cracks themselves (independent of the frost resistance of the concrete). Peter.15. Earl L. (AC1 201. “Tests on Reinforced Concrete Beams Under Long-Term Loads (Dauerstandversuche mit Stahlbetonbalken). 5. Concrete subjected to water soluble salts should be air entrained [6.5 percent for F/2 in.5 to 5. Possible solutions to these problems include limitations on reactive constituents in the aggregate.16 the possible hazard of using calcium chlo- ride in a water-soluble salt environment warrants a recommendation against its use under such circumstances. calcium chloride or sodium chloride should be used and only within recommended application rates. and Ruhle. “Development and Distribution of Cracks in Rectangular Prestressed Beams During Static and Fatigue Loading. pp..g. ability of cracks to drain. The tensile splitting strength can be as high as 1550 psi (10. or addition of a satisfactory pozzolanic material. Oct. 331-358.. Joe W. it is expected that structural elements made with polymer modified concrete will exhibit superior serviceability in cracking.Use of polymers in improving cracking c h a r acterisitics Extensive work is available on the use of polymers in modifying the characteristics of concrete. 538-546.” Proceedings Institution of Civil Engineers (London). the use of calcium chloride in reinforced structures exposed to unusually moist environments is to be avoided regardless of the presence or absence of water-soluble salts in adjacent waters and soils. 13. Field exposure tests of reinforced concrete beams5*17 (subjected to freezing and thawing and an ocean side environment) indicate that the use of air-entrained concrete made the beams more resistant to weathering than the use of nonair-entrained concrete.2. Based on reports of AC1 Committees 201 and 212 9 5. ground water.2R).2R and Reference 5.. 24. creep. Seventh IABSE Congress (Rio de Janeiro. V. 5. No. and permeability. International Association of Bridge and Structural Engineering. The lack of such resistance may be due to either the use of non-frost-resistant coarse aggregate or the failure to produce a satisfactory air-void system or failure to protect the concrete from freezing prior to the reduction of the freezable water content by maturity to a tolerable range. Beams with modern deformed bars were found to be more durable than those using old-style deformations. is not reduced by pozzolanic admixtures.” Journal. Sharma. When such applications are necessary. G. Zurich. No.4.” ACI JOURNAL . Mar. On the otherhand. typical time of drying after wetting before freezing). etc. e. 36-51. Lutz. Brendel. shrinkage. 5.. the concrete durability will better.5.. The initiation of D-cracking near joints or other cracks in pavements is a good example. should have adequate cover (about 2 in. II. 5. when this might not occur in the absence of such cracks. LeRoy A.5 percent for normal 3L4 in. preexisting cracks may also function to allow concrete to dry below critical saturation before freezing. H. 56A. Edge curling and delamination 5. Durability of Concrete Construction. 1969.A Test Report.3 of many “layered” concrete systems have shown that differential shrinkage cracks are by far the most common and most likely to increase and widen with time. No. Schrader.” Document D2. Proceedings V.1 Bond to underlying concrete . Cement and Concrete Association (London). 2.21. Stevens. A. Beach Erosion Board.. 53..” Technical Report No. American Concrete Institute. No.17. Reflective cracking (stress cracking) 3.Control of cracking in concrete layered systems* 6. 5. 3. R. Rolf. W.. Gallus. J. N. Mather.” Causes. 1968. Holmberg. 35-44. Washington. V.13. Illston. 1970. Stockholm. Ake. Atimtay. Mar. warehouse floors. D. etc. S. Phil M.C. No. pp. A.6. 187 pp. 420 pp..Fiber reinforced concrete (FRC) overlays When properly proportioned. and Control of Cracking in Concrete. pp.10.” BNL Report 50134 (T-5091. Roshore. 1973. “Concrete in the Oceans . Holmberg. Dec. June 1973.22. pp.” The Structural Engineer (London). and Eligehausen. Dec. Polymers in Concrete. National Swedish Council for Building Research. 1978. 7. “Early Chloride Corrosion of Reinforced Concrete . pp. 170-174. 253-257. “Cracking Induced by Environmental Effects. Mechanism. E. etc. Fibrous concrete overlays of highways. 11. pp. The major sources and types of cracking in these layered concrete systems are: 1. pp.15.CONTROL OF CRACKING 224R-23 5. No. John A.” Beton und Stahlbetonbau (Berlin).” ACI JOURNAL .12.1-6.. Differential shrinkage cracking 2.6*2-6*7 6. walkways. and Dave. 1. R. 1972. 5. Detroit.During early fibrous concrete overlay work. W. pp. V. 5.” Proceedings.. and placed. Polymers in Concrete. it was thought that a “partially bonded” layer was the ideal system. Some typical observations for similar field or laboratory conditions are discussed below.16. 1968.19. Brookhaven National Laboratory. Proceedings.2. walkways. Edwin C.. May 1967. 1977. 5. A. 77-81.Cracking and Corrosion. fiber reinforced concrete (FRC). Monograph No.14. and Stevens. 5. 5. and generally left in damp condition. V.. 5. strengthening and/or renovation of concrete pavements. 41-50. SP-20. Part 2. 487-496. Detroit. 47 No. 1978. 5. American Concrete Institute. V. “Tests on Prestressed Reinforced Concrete Beams.. No. “High Strength Field Polymer Modified Concretes. The term “partially bonded” means that no deliberate attempt is made to bond or to debond the topping layer to the underlying material through agents. After the evaluation of partially bonded projects. 5. “Field Exposure Tests of Reinforced Concrete Beams. have been used since the early 1970s. 5.” The Structural Engineer (London). Bryant. mixed. “Lapped Splices of Deformed Bars Under Repeated Loadings (Ubergreifungsstosse von Rippenstahlen unter nicht ruhender Belastung).” Dansk Selskab for Byaningsstatik (Copenhagen). 96. and Sauer. Ukadike. 5. American Concrete Institute/Iowa State University. 6. 64..2 . SP-40. V. airfields. Beeby. pp. ASCE. pp. more commonly referred to as latex modified concrete (LMC). Sept. “Concrete Polymer Materials. 5. “Test Performances and Design of Concrete Beams with Limited Prestress.” Proceedings. and Lindgren. A “layered” system can also be created by impregnating the upper portion [l/z to 3 in. Dec. 5. Bryant.. etc. . 3. June 1957.. Detroit.. Edward G.18. or internally sealed concrete. “Factors Affecting Durability of Concrete in Coastal Structures. Fibers are usually steel with lengths between 10 and 60 mm (l/2 to 2l/2 in. “Long-term Cracking in Reinforced Concrete Beams. The effects of fibrous concrete on cracking in a “layered” system depend largely on the field conditions of each situation. 9. Proceedings V. W. The use of “layered” concrete systems has been increasing during the last 10 years in the renovation of deteriorating bridge decks. 103.” Concrete (London). 1978. Nawy.20.” Technical Memorandum No. 70. American Concrete Institute. 5. pp. Incorrect construction practices Long term observations 6. pp. “Crack Width Prediction and Minimum Reinforcement for Crack Control. SP-58..” ACI JOURNAL. 4.1 . Nov. 2307-2322. 57 pp. V. fasteners. 362 pp. ST12. 1. 12. Rehm.. Broms.).5. No. Maurice M. F. 62. Chapter 6 .Introduction A “layered” concrete system can be created by a mortar or concrete overlay (topping) placed on an existing concrete surface. 5. 67-72. 1968.9. 1977. Ergin. Differential temperature cracking 4. Bennett. 457-462. The overlay can be portland cement low slump dense concrete (LSDC).7. J.” ACI JOURNAL . 1965. a crack resistant topping layer of FRC can be the solution to certain field problems. M. (10 to 80 mm1 ] of existing concrete with a monomer system that requires polymerization after soaking. “Corrosion of Reinforcing Steel in Concrete and Its Relation to Cracking. Woods. . The surface to be overlaid is cleaned of all loose material. warehouse floors. 44. V.8. Jan. 445-459. Bishara and Ernest K. F. Mather. 5. Institution of Civil Engineers (London). “Crack Spacing and Crack Width Due to Normal Force or Bending Moment. this procedure has become the least desirable technique to *Principal authors: Alfred G.. “Technique for Investigation of Internal Cracks in Reinforced Concrete Members. polyethylene sheet. 1973. and in new two-course construction of decks and pavements. Hubert..11. polymer-portland cement concrete (PPCC). Beeby. Detroit. usually by hosing. 5.. 606-611. 1969. No. Bengt B. and Ferguson. 5. (20 to 40 mm)] have been used in the renovation of deteriorated bridge decks and in new two-course construction to effectively resist the penetration of chloride ions from deicing salts and prevent the subsequent corrosion of the reinforcing steel and the spalling of the concrete deck.” This post-cracking strength is one of the most important characteristics of FRC. or potholed surfaces a thin leveling and debonding layer of sand or asphalt is desirable.2.9 percent fibers by total volume will provide substantial crack resistance. the mix water can be held to reasonable levels.11.6. As fiber diameter increases for any given volume percentage. For concrete with 10 mm (3/8 in. Longterm evaluations are currently underway.6 Joint overlays .6.3 6. Another technique.There has been considerable discussion about the condition and effectiveness of steel fibers that bridge over or through a crack.4 Fibers in open cracks . that if the cracks are tight (0.2 Fiber size and volume .001 . If the base slab is relatively crack free.)]. If fiber contents much greater than these are used. or if aggregate gradations are not suitable. but the two types do exhibit some difference in physical crack behavior.8 percent is adequate. If possible. 6. spalled.003 in.3R provides detailed information on suitable mixture proportions for steel fiber reinforced concrete. 6.) aggregate about 1. Also. about 0.6.4 to 1.) aggregate. (0. The failure (crack) zone for glass is more localized. The high water content provides the basic ingredient for shrinkage cracks.3 Fiber type and shape . the number of small cracks decreases. The asphalt itself will act as a debonding layer if it has a reasonably smooth surface without potholes. This type of construction lends itself particularly well to deteriorated airfield slabs which have been resurfaced with asphaltic concrete but require additional rigid pavement to take increased loads imposed by heavy aircraft. Inspections of a large number of bridge decks overlaid with LMC6.08 mm)]. and for mortar. 6. the crack arresting mechanism is limited. Also. which has been used when the base material to be overlaid is reasonably smooth.224R-24 ACI COMMITTEE REPORT use. a bonded layer with matched joints is generally the best approach.6. The basic crack theory is applicable to both glass and metallic fibers.8 Although fiber size and volume have little effect on the formation of the first crack they are major factors influencing subsequent crack development. even after several years of exposure.Latex modified concrete (LMC) overlays Latex modified mortar and concrete bonded overlays [3/4 to 1 l/z in.2. Over a period of several years many partially bonded FRC overlays have shown noticeable amounts of reflective cracking and edge curling.2 in. For concrete with 20 mm t3/4 in. most have been unsuccessful.03-0.0.1 have indicated that there is a .Because of their increased resistance to pullout. Through the use of water reducing admixtures. if joints in a base slab are overlayed with FRC without taking special design precautions to prevent reflective cracking. The water requirement for fibrous concretes is higher than that of normal concrete due to the high surface area of the fibers.2 percent is normal.3. deformed steel fibers have an advantage over smooth ones with regard to both pre.6. a totally unbonded overlay is generally best where severe cracking is present or may develop in the base slab.12 Some of these decks have been in use for over 10 years. Essentially unbonded systems have been constructed satisfactorily where FRC is placed over an asphalt layer. the overlay will crack at joint locations. If the FRC layer is of sufficient thickness.6. The “obvious” problem is that after cracking.6-gp ‘JO If possible. On irregular. but the number and width of larger cracks increase. 6. as the volume percentage decreases. Regardless of the reason. the majority of investigations 6.2.and post-cracking behavior. steel fibers will oxidize and provide no long-term benefit. At the time of cracking. these admixtures should be used to adjust the mix proportioning for a bonded overlay so that the water/cement ratio and cement factor approach the same values as used in the underlying material. consists of placing the FRC over a layer of polyethylene sheet.7 As with conventional concrete overlays. 1. the fibers lose their bond to the concrete but continue to provide a “mechanical resistance to pullout.5 Mix proportion conditions -ACI 544.2.6 have shown. or if the overlay is of sufficient thickness and strength to resist the extension of cracks in the original slab. The curled edges are typical in thin overlays [less than about 3 in. microcracking in the general vicinity of a major crack is typically more prominent with steel than glass.6.3 . the fibers will not oxidize. Test+* have shown that glass FRC has less ability to store energy after its failure in flexure than steel FRC. the overlay should have aggregates of similar physical properties unless the original aggregates are unsuitable. the number of fibers decreases and the spacing between fibers increases. the spacing increases.5.2. the advantage is not always worth the additional expense. as the fiber spacing increases. However. 6. high cement and water requirements result and the FRC layer is susceptible to shrinkage cracks. (76 mm)] and can result in cracks if subjected to long-term dynamic loading. However.The crack arresting mechanism on which the basic theory of FRC is founded depends on fiber spacing. If the fiber spacing becomes relatively large [more than about 5 mm (0.Different methods of joint overlaying have been tried. C. Oct. during the soak period and after cooling. References 6. Ernest K. (75 to 100 mm) to reduce differential shrinkage and the high incidence of random cracking. If a more viscous monomer is used. The surface of the underlying concrete should be cleaned by sand blasting to assure adequate bonding with the overlay. thermal expansion. traffic is normally diverted to one lane. To reduce air pollution. coarser particles of the mixture which cannot be scrubbed into immediate contact with the surface of the underlying concrete.6. D. Champaign.9 to 1. particularly in urban areas. In this case.2. and no mending will occur. the monomer can run out of the crack before it is polymerized. 6.2 m) apart.3. 1979. Restraints. Anthony V. “Fiber Reinforced Concrete . while renovation and application of the overlay proceed on an adjacent traffic lane. preventing movement at the perimeter of the concrete to be polymerized.S. or when appropriate actions are taken for cold-weather concreting (ACI 306R) or hotweather concreting (ACI 305R). and the monomer system to prevent the occurrence of unmended cracks. the depth of penetration into the concrete will be adversely affected. Transverse cracks. thermal expansion will offset drying shrinkage until the concrete cools. “Deck . should be avoided. should be removed.Field Performance Analysis.14..15 There has been considerable discussion about this procedure due to observations of cracks during or immediately after the drying step of these p r o j e c t s . Bishara.. To keep the bridges in service. To reduce the incidence of cracking and sub sequent loss of latex modified concrete overlays it is recommended 6 . 6. and the crack will not mend. This type of cracking is not as extensive in new two-course construction. 2. However. B. the overlay will have to carry a portion of its own weight and may crack in negative moment regions. Schrader. 1972.. there may be a relationship between the degree of transverse cracking and the intensity of heavy truck traffic during reconstruction.Polymer impregnated concrete (PIC) systems Surface impregnation and polymerization of concrete in place is a relatively new process but has been used successfully in a number of field projects. Washington. A. after drying has been completed.6. also appear on many of the bridges inspected. the overlay should be placed after removing the forms from the base concrete. 97 pp. Federal Highway Administration.CONTROL OF CRACKING 224R-25 high incidence of fine. Overlays should be placed only when the ambient weather conditions are favorable.A General Discussion of Field Problems and Applications.13. The strain capacity. in lieu of sand blasting. Apr. the presence of moisture prevents the monomer from entering the concrete adjacent to the crack. Temperatures during drying are usually in the range of 120 C (240 F) to 150 C (310 F) for about 4 to 12 hr. The long-term influence of polymer impregnation on the behavior of cracking in concrete is not known at this time but will be established by the evaluation of currently completed field projects. random.1 that: 1. 4. The slump of latex modified concrete mixtures should be between 3 to 4 in. 6. high pressure water jet cleaning [5000 to 6000 psi (35-40 MPa) at the nozzle] may be used just prior to placement of the overlay. so that it does not drain from the crack. spaced 3 to 4 ft (0. although extensive data do not exist linking the effect of traffic-induced vibrations during reconstruction to deterioration or cracking in bridge decks.4 . shrinkage cracks in a large portion of the renovation jobs. In new two-course construction. If placed before the forms are removed. so that stresses caused by the weight of the overlay are born by the underlying concrete. Gray. the monomer will fill any cracks that have been created in the top surface of the concrete due to drying. 6. 6. and Munch. as defined in ACI 308 on curing.6.. “Latex Modified Concrete Bridge Deck Overlays . To some extent. The cracks will be mended when the monomer is polymerized. If traffic must be maintained. The finishing equipment should have been proven to be effective for adequately placing the concrete to the required density. The quality of the overlay may be affected by the movement of the deck. consideration should be given to placing overlays when traffic is low and/or when vehicle speed is restricted. The engineer should thoroughly evaluate all effects of the drying cycle in a PIC project and plan the drying temperatures and duration. and specific heat of the material should be considered. 3. If there is a water source behind the material to be polymerized it is possible for moisture to re-enter the crack.” Report No. 5. I n t h e c a s e s t h a t h a v e b e e n evaluated.” Technical Manuscript M-12. the cooling cycle.15 the cracks were determined to either have been in the concrete prior to the impregnation or they were caused by improperly controlled drying during initial stages of the impregnation procedure. U. G. If a crack is open and can drain (as is the case with vertical surfaces and cracks through the full depth of a slab). A thin coating of the overlay mixture should be thoroughly scrubbed into the surface of the underlying clean concrete immediately before placing the overlay mix to increase the bonding between the layers.1. FHWA/OH/79/004. but before the monomer soak starts. H. Ideally. Army Construction Engineering Research Laboratory. G. Smoak. the design stresses will be high and more cement must be used to provide the stipulated factor of safety. U. a high coefficient of thermal expansion. thermal insulation has been used to protect exposed surfaces.11. Bishara. 238 pp.8. “Time to Corrosion of Reinforcing Steel in Concrete Slabs. a specialized type of mass concrete. The degree of temperature control necessary to prevent cracking varies greatly with such factors as the location. In the case of a dam. Champaign.S. and Rice. 19751.... Transportation Research Board. Prepared for Federal Highway Administration.6.10. John L.Construction Material for the Seventies..” Report No. Fowler. S. John L. 6.. 1977. and more *Principal authors: Donald L. pp. Champaign. Gray. U. the creep (or relaxation) of the concrete is important. If the dam is very high. 6.. the ambient temperature variation alone may be sufficient to cause cracks to form at exposed surfaces. W. bridge and building foundations.S. H. ASCE. “Fibrous Concrete McCarran International Airport. “Recent Developments in ConcretePolymer Materials. May 1971. A. Monroeville.. 6. A.” Interim Report No. Often at high altitudes. navigation locks.. B. K. “Polymer Impregnation of New Concrete Bridge Deck Surfaces. 6..Control of cracking in mass concrete* 7. a high modulus o f elasticity. where a concrete has a low tensile strength. the two factors which govern the strain capacity are the modulus of elasticity and the tensile strength. 1975. Army Construction Engineering Research Laboratory. P.2. For strain which is applied slowly. “Utilization of ‘Wirand’ Concrete in Bridge Decks. in the extreme case. 1976. Houghton and Roy W.15. J.Introduction Temperature induced cracking in a large mass of concrete can be prevented if proper measures are taken to reduce the amount and rate of temperature change. pp.224R-26 ACI COMMITTEE REPORT Slab Repaired by Fibrous Concrete Overlay. Jan. Ernest K. No. M-134. “Fibrous Concrete Pavement Design Summary. the properties of the concrete and the external restraints. U.9.” Report No. Rodney J. 6.” Transportation Research Record. only in this case the temperature variations are seasonal. Chicago.. post-cooling or a combination of the two. 1975. June 1975.. 9 pp. has been amply covered in Chapter 8 of ACI 304 and will not be discussed in this report. Rice. U.. “Fibrous Concrete for Pavement Applications.13. R. Army Construction Engineering Research Laboratory. B. Denver.. Mar. Champaign. it may crack when there is a quick drop in . 6. and Stebbins.” Proceedings. the higher dam will have greater horizontal dimensions which cause greater restraint and the need for still closer temperature control. “Fibrous Concrete .5. June 1975. A. EES 435 (ODOT-12-74) Ohio Department of Transportation. Williamson. “Bridge Deck Rehabilitation with Steel Fibrous Concrete. Dec. 6.1 . and Tantayanondkul. 1974. Pa. Washington. powerhouses. These surface cracks continue inward with only approximately half the stress which is necessary to cause internal cracking. U. Kaden. Thus. G. P. Tremie concrete. and Lankard.. 6. and Batson.. pp. and Naaman. Richard A.7. 1978. C01. Also. the height and thickness of the structure. Bureau of Reclamation. Another important property of concrete is the coefficient of thermal expansion. M-13. and is fully restrained. D. but with some aggregates.. E. “Mechanical Properties of Glass and Steel Fiber Reinforced Mortar. Chapter 7 . American Concrete Institute. W.” Second International Symposium on Concrete Technology (Monterrey. The average coefficient of thermal expansion of mass concrete is about 9 millionths per deg C (5 millionths/F). This makes for more heat generation and a consequent tendency toward higher internal temperatures. D.. rather than daily. Mexico. Clear.” Polymers in Concrete.” Report by General Analytics. 6. May 1972. Denver. Concretes differ widely in the amount of tensile strain they can withstand before cracking. The location of the structure affects the degree of temperature control which will be required. Although a large amount of the data for this chapter has been obtained by experience gained from the use of mass concrete in dams.” Las Vegas. Walker. Battelle Columbus Laboratories. David W. 6. 6. G. The factors affecting strain capacity and creep rate are discussed more fully in Section 7. Shah.4. for Battelle Memorial Institute.S. The amount of strain which a temperature change will produce is directly proportional to the coefficient of thermal expansion of the concrete..14.. 1972. University of Illinois. the character of the aggregate. Hefner. 179-196. “Polymer Impregnation Used in Concrete Repairs on Cavitation/Erosion Damage. Measures commonly used include precooling. Carlson. the height affects the need for crack control. Gray. V. B.” Conference Proceedings M-28. G.C. 16-24. Mar. C. Aug. Generally at high altitudes the daily variations in temperature are greater than at low altitudes. the coefficient may be as high as 15 millionths or as low as 7 millionths (4 to 8 millionths/F). FHWA-RD-75-72. 1977).12. Detroit. R. 13 pp. S. Apr.. The properties of the concrete affect the problem of crack control. SP-58.S. 500.” Technical Report No.. Depuy. 225-248. The Ohio State University. etc. 102. 1974. Army Construction Engineering Research Laboratory. H. For strain which is applied rapidly. Nevada.S. A similar condition is likely to be found when a structure is located at a high latitude.. “Use of Latex in Concrete Bridges Decks. 6.” Department of Materials Engineering. G.” Presented at the Third International Exposition on Concrete Construction (New Orleans. recently. it applies equally well in mass concrete used in other structures such as steam power plants. 1974. Bureau of Reclamation. Schrader. In the case of Dworshak Dam.2 . the creep is low and vice versa. assume that the aggregate and the cement paste have the same modulus of elasticity. the strain capacity will be high. or using other specialized procedures. For such concrete. to very costly measures. Tensile strength is also important. The second measure to prevent cracking is to control the factors which produce tensile strain. When compressive stress is applied. Many tests on very lean concretes. creep tends to increase the strain capacity. the two factors governing the tensile strain which a concrete can withstand are the tensile strength and the modulus of elasticity. More data on the thermal expansion of concrete may be found in the reports of ACI Committee 207 (ACI 207. and for this reason. crushed aggregates are apt to be superior to natural aggregates for crack prevention. The insulation is left in place long enough to permit the concrete temperature at the surfaces to slowly approach the ambient. The lost stress must be shifted to the aggregate to maintain equilibrium.1R and ACI 207. can undergo a large strain before failure. even when fully restrained. the strain to failure was almost three times as great for strain applied over 2 months as for quickly applied strain. for a dam near the equator with favorable aggregates. restricting the maximum aggregate size. This imposes an elastic strain on the aggregate which accounts for a large part of the creep of the concrete. or it can be determined on concrete beams located at the third points. This may require careful aggregate selection. When strain is applied slowly. or the greatest tensile strain capacity. in a location where temperature variations are great and where the only economical aggregates have high elastic moduli and high thermal expansion. The first is to modify the materials and mix proportions to produce concrete having the best cracking resistance. First. As stated above. Thus.Crack resistance The tensile strain which concrete can withstand varies greatly with the composition of the concrete and the strain rate. If concrete contains rough textured aggregate of small maximum size. using the minimum cement content for interior concrete. post-cooling. for example. From these considerations.7.1 A high creep rate of concrete is helpful in preventing cracking when the tensile strain is applied gradually. a higher maximum temperature. The aggregate does not creep under moderate stress but the paste does. Thus. This may mean precooling. present practice calls for both precooling and post-cooling. then. insulating (and possibly heating) the exposed surfaces of the concrete during cold weather and designing to minimize strains around galleries and other openings. the strain capacity is far greater than when the action is rapid. Strain capacity can be measured directly on cylindrical specimens loaded in tension. some concretes can withstand a quick drop in temperature of as much as 10 C (20 F). such as are used for the interior of large dams. 7.2R). if the size is reduced too much. an increase in the rigidity . and the paste which is between aggregate particles relaxes and loses stress. In the latter case. The amount of this elastic strain is directly related to the modulus of elasticity of the aggregate. the stress and the corresponding strain will be the same in the aggregate as in the cement paste. it is apparent that the degree of crack control necessary for the safe elimination of joints may vary from nothing at all.1 The creep of concrete under sustained stress is affected by the stiffness of the aggregate. The importance of aggregate rigidity on creep of concrete may be illustrated by two examples. Additional research into the most effective use of thermal insulation is needed particularly for regions having severe or sub-arctic climates. Next. On the other hand. the gain through greater strain capacity of the richer concrete with smaller aggregate may be more than offset by the greater strain that must be withstood. When compressive stress is applied. Smaller aggregate requires more cement for a given strength which results in more heat.7. the lower the creep.CONTROL OF CRACKING 224R-27 temperature of only 3 C (6 F). For many purposes. assume that the aggregate has a much higher modulus than the cement paste. There are two measures which can be taken to provide safety against cracking. or until additional concrete is placed on or against the surface being protected. When the modulus is high. The elastic strain in the aggregate due to the creep of the paste will then be less than it was when the moduli were equal. the average stress in the aggregate will be higher than that in the cement paste and the paste will creep less than it did when the moduli were equal. the tensile strain which the concrete can withstand is approximately equal to the tensile strength divided by the modulus of elasticity of the concrete. and greater subsequent strain due to cooling. concrete in the interior of a large mass which must cool slowly. the more rigid the aggregate. there is an optimum with respect to the aggregate size. Thus. have shown that tensile failure occurs without much “plastic” strain when loading is applied rapidly. However. Since the tensile strength of concrete is nearly independent of prior loading. it is sufficiently accurate to assume that the tensile strain capacity is inversely proportional to the modulus of elasticity of the concrete. It follows that the modulus of elasticity of the aggregate is important because of its large effect on the deformability of the concrete. and for the application of thermal insulation to exposed surfaces during cold weather. More than the necessary amount of cement is a detriment rather than an advantage. First. the strain is simply the temperature drop multiplied by the coefficient of expansion. 4. Concrete with large tensile strain capacity.2R). This may mean limiting the maximum aggregate size to a value somewhat below that which might be the most economical otherwise. Absence of stress raisers. Thus. to very expensive measures. For information on other methods of predicting temperatures in mass concrete. the control of boundary strain is sufficient to prevent cracking. Short blocks. but is not readily available). ASTM Type II (moderate heat) cement should be used for mass concrete construction (Note: Type IV. The finite element method can be used for the prediction of temperature distribution. In this case. which often depend upon the ambient temperatures. even if large. The prediction of probable strain requires the prediction of the temperature to be expected.4 . Low cement content (permitted by low design stresses). temperature change is the main contributor to tensile strain in mass concrete.224-28 ACI COMMITTEE REPORT of the aggregate acts in two ways to reduce the creep of the concrete. 7. by using a water-reducing . This prediction can be made quite reliably if the adiabatic temperature curve for the concrete is known. The concrete with less water and cement is superior in two important ways: it undergoes less temperature change and less drying shrinkage.5a 7. Low degree of restraint. Small daily and seasonal temperature variations. In some cases. Internal strains usually develop slowly enough to be tolerable. up to 35 percent or more of the cement can be replaced by an equal volume of a suitable pozzolan and still produce the same strength at 90 days or 1 year. diatomaceous earth. High yearly average temperature. 7. Pozzolans can be used to replace a portion of the cement to reduce the peak temperature due to the heat of hydration (207. preference should be given to that which yields best crack resistance. boundary temperatures and dimensions. Some of the conditions which facilitate crack prevention are: 1. as well as the thermal diffusivity. also. the determination of probable tensile strain is the next step. Drying shrinkage is important only because it may cause shallow cracks to occur at surfaces. The heat producing characteristics of cement play an important role in the amount of temperature rise. The analysis must include many steps of time to properly account for the creep (or relaxation) and the different and changing properties of every lift of concrete. Descriptions of test methods suitable for measuring the physical properties necessary for the prediction of temperatures and strains are given in Section 7. The lowest practical cement content permitted by the strength and durability requirements should be used to reduce the heat of hydration and the consequent thermal stresses and strains.5. Some of the more common pozzolans used in mass concrete include calcined clays. or in portions of structure well removed from restraining foundation. 6. Where several sources of aggregate are available economically.4 The main problem is that of choosing the correct boundary temperatures. and availability. a reduction in the water content of concrete permits a corresponding reduction in the cement content. volcanic tuffs and pumicites and fly ash. such as galleries. This list suggests many of the measures which can be taken to prevent cracking.Control of cracking Given the probable temperatures and strains. 3. This is important. see the report ACI 207.3 . where conditions are unfavorable. a thorough analysis is laborious because of the time-dependent variables. recommended. 9. because in many cases. cost. This can be accomplished using finite element computer programs. In general. Minimum water content can be achieved by such measures as specifying powerful vibrators which permit low slump. The actual type of pozzolan to be used and its appropriate replacement percentage are normally determined by test. The preventative measures will vary from nothing where weather and materials are favorable. 2.7. 8.Determination of temperatures and tensile strains Tensile strain in mass concrete results mainly from the restraint of thermal contraction. Low casting temperature. strains near a boundary due to brief thermal shocks can be computed quite readily because in such cases the concrete can be assumed to be fully restrained. Cement of low heat generation. low heat cement is. and to a lesser degree from autogenous shrinkage. 10. 5. usually this will be a crushed material of low thermal expansion and low modulus of elasticity.lR.7. as with yielding foundation. an attempt should be made to produce a concrete with large tensile strain capacity. Slow rate of construction when no cooling is used. On the other hand.3a 7.6 Even with the finite element method. the designer must determine what measures are most practicable to provide ample safety against cracking. After the predicted temperature history is known. 7. It is often satisfactory to use air temperatures found in weather reports as the surface temperatures to be used in the computations. and.Thermal diffusivity and thermal expansion are important in the control of cracking due to temperature change.Creep may be defined as the continued deformation of concrete under sustained stress. rollon flexible rubber type material. appears to have potential for the insulation of concrete lift joints during the active construction season.1 Adiabatic temperature rise . In extreme environments. One measure which offers promise is that of placing crack resistant concrete at boundaries (sides and top of lifts). It is easy to install and remove and can be reused many times. which make use of very large buckets. in some cases. it appears more promising to use precast concrete panels for forms and to leave these panels as a permanent part of the structure. Precautions must be taken against using too much insulation or leaving it in place too long.5. it can be timed to remain effective on the lift joints for approximately the period of time between successive placements and be easily removed by a final washing prior to placement of the new lift. 7. More information on the use of precast panels for protection of mass concrete can be found in ACI 347. Thermal insulation on exposed surfaces during cold weather can protect concrete from cracking. cause the interior temperature to begin to increase again. and foamed sprayon material which becomes semirigid in place. Spray-on insulation of timed longevity for frost protection of agricultural plants and trees. Spray-on insulation can be used on either horizontal or vertical surfaces. 7. The panels would then serve as both forms and face concrete.8 through 7. it can be used to this limited extent without serious effect on economy. This insulation can be formulated to disintegrate at a given time after application. subsequent.5 kg cal/m’/hr/C (0. Therefore.Testing methods and typical data 7. A standard test for creep of concrete in compression is detailed in ASTM C 512-76. it may be necessary to remove the insulation in stages as the warmer months approach. also. Experience has shown that insulation which permits transmission of light rays should not be used because a temperature rise occurs between the insulation and the concrete when the insulation is subjected to direct sunlight. and their determination is detailed in References ACI 207. .6. Precast panels made of low conductance lightweight concrete or regular weight concrete cast with laminated or sandwich layers of low conductance cellular concrete also are acceptable as a means of insulating the interior concrete.CONTROL OF CRACKING 224R-29 agent. Temperatures within the concrete just below the insulation should be allowed to slowly approach the environmental temperature.75 to 0. this partial measure may make the whole structure crack free.1R and 7. if the concrete has a very slow relaxation rate (or creep rate) the amount of insulation and the long protection time required may make this measure impractical.6 to 0. as currently used for concrete. The concrete can relax as rapidly as the tensile stress tends to develop. the tensile strain need never exceed the dangerpoint.1R.10.2 Thermal properties of concrete . The semirigid panels are usually installed on the inside face of the forms. Temporary anchors embedded in the newly placed lift of concrete retain the insulation on the concrete surface when the forms are lifted. which could result in stopping the desired cooling of the interior mass. Since most cracks originate at boundaries. Fig.’ l5 Creep of concrete in tension is difficult to measure. Roll-on insulation is particularly applicable for use on horizontal lift joints. and by placing the concrete at a low temperature. The reader is referred to ACI 207. However. These panels should be of good quality for durability.5.3 Creep of concrete . This type of insulation is particularly useful for increasing the thickness and effectiveness of insulation already in place and for insulating forms.1R for methods of test. until finally. 7. and preferably lightweight so as to provide good thermal insulation.1. That report gives data on adiabatic temperature rise of concretes having a single cement content but having different types of portland cement.1 gives typical adiabatic curves for Type II cement and various quantities of cement and pozzolan.5 . Insulation. Even though the more crack resistant concrete may be too costly to be used throughout the structure. Thus. 7. where large amounts of insulation will be required during severely cold months. The insulation is easily removed from the surface when desired. if enough insulation is used and it is left in place long enough. But thin layers of concrete next to the forms cannot be placed easily with present-day construction methods.1 represent data from mixes containing equal volumes of cementitious materials (cement plus pozzolan) thereby showing the effect of pozzolan replacement of cement on temperature reduction. If the insulation is sufficient to allow slow cooling. Details on pipe cooling are given in Section 7. stable temperatures are reached. The approximate range of thermal properties is shown in Table 7.10 BTU/hr/sq ft/F). Precooling the concrete during its production and post-cooling it with embedded pipe systems after it is placed are especially effective measures. This is to prevent the occurrence of thermal shock which could induce cracking at the surface with possible. can be obtained in a variety of forms and materials having practical installed conductances ranging from 3. 7. deeper propagation into the mass.5.The temperature rise which would occur if there were no heat loss is defined as adiabatic temperature rise. It can be obtained in semirigid board type panels. Curves A and B in Fig. 2 to to 8 14.00 141 2.4 2.1 .66 46.Portland Cement 306Ib/cu yd(l8l kg/~).-_.-_. PDZroh-50 Ibhu yd(30 kg/m) l l -5 Type II Cement 0 0 4 8 I2 Age ..l---- ..22 - Per O F Per O C _ --__ -_ - -~-~ 4 7.56 kg/cm’ psi -3 -6 x10 -3 x 10 x 10 ~-_.l%zzoIon-74lb/cuyd(44kg/m) Curve C ..__---_ 0._. millionths __ Thermal properties Diffusivity Conductivity _____--_.Pozzolan-63Ib/cuyd (37 kg/m) Curve D . 7.--_----_.._.15 to 0. I81 Ib/cu yd (107kg/m3). x 10 ..Illustrative range of thermal and elastic properties of mass concrete __-___-____~_-__~_--__Coefficient of linear expansion..__--_.-_------ psi -6 1 day ---___-_--. Days Fig. 28 days 90 days 3 days 7 days __YPW ___--- __~---_.~-___--_-.6 5.__--180 4.224R-30 ACI COMMITTEE REPORT 50 - A 40 c _.Pozzolan-None l .-__----_----__ ft x hr x O F m x hr x O C-_.7) I6 20 24 -0 28 TABLE 7.____.. IO Curve B .25 1 .1 .Typical adiabatic temperature curves for mass concrete (Reference 7. _~____ _ _.Portland Cement 214 Ib/cuyd (127 kg/m3).ft’ hr Specific heat ppp BTU/lb O F or Cal/g O C 0.5 -_.-- 0. _ ~_----_ ----.067 I ____------__~--~-._ 20 1 0 D I LEGEND Curve A .Portland Cement 148 lb/w yd (88 kg/d).00 281 psi.040 to 0.----~kg/cm’ psi -6 x 10 x 10 x 10 x 10 x 10 -_----_L--_----_.00 kg/cm’ x 10 -3 352 Poisson’s Ratio 0.Portland Cement . psi kg/cm’ kg/cm2 -3 -6 -3 Elastic properties ~_~_____~---_~-----_ Static modulus of elasticity (E) for age _ of test indicated _----____-----p __--__~-~-_--_ . 7. Shown in Table 7.5 0. (1+1) -7Dg E =0.5 Autogenous volume change . For example. but loading at the early age of 1 day is not always practical.4 2. Specimens should be loaded at the same ages as specified for the modulus of elasticity tests.7.6 Tensile strain capacity-.0579 L O G . although any reliable method of measuring strain can be employed.5.209+0. (30 x 30 cm) to 24 x 24 in.2 are values for sustained modulus of elasticity E. concrete 2 days of age loaded at age 1 day would have a sustained modulus of elasticity (E. the specimens should be large enough to permit concrete very nearly like that to be used in the structure. Cylinders of 9 x 18 in.17 MPa) fiber stress per week for rapid and slow loading tests. which in turn are used to develop tensile stress coefficients per degree temperature drop for the condition of full restraint. 7.46 kg/cm2/C) for each degree drop in temperature (see Table 7.6 0. Measurements are begun as soon as the specimens are hardened and sealed. (160 to 325 cm) long are generally used. 7.(t+l) Days Specific Creep Only Fig. The concrete test beam used for determining the strain capacity should be protected during the test to prevent loss of moisture by wrapping it with an impermeable material. Table 7. (38 mm) maximum aggregate are frequently used.Autogenous . (76 mm) maximum sized aggregate or 6 x 16 in (15 x 40 cm) cylinders with 11/2 in.5 = 0. Fig. since the measured strain in a beam which is gradually loaded from the age of 1 month.7. but neoprene should be avoided because it allows some moisture to escape. The loading rates are generally 40 psi (0. moisture or stress. is only about 10 percent more than that computed using creep data as obtained from similar concrete in compression.The tensile strain ca- pacity tests are generally performed on unreinforced concrete beams under third-point flexural loading. also.13 is the expansion or contraction of the concrete due to causes other than changes in temperature. Such an assumption can be considered as reasonable when the stress is low.7. it appears permissible to apply compression creep data to concrete stressed in tension in cases where approximate results will suffice. The strain for rapid loading can be measured using either surface or embedded strain gages or meters.66 psi x lo6 (46.1 shows values of the modulus of elasticity of a particular concrete after various ages of curing.7. not only does the instantaneous deformation increase.This subject is treated in detail in ACI 304. but a contraction increases in tendency to crack.4 kg/cm2 x lo31 (see Fig.28 MPa) fiber stress per minute and 25 psi (0. 7. Butyl rubber is satisfactory for sealing the specimens.1 and 7. embedded meters are best.2 illustrates important computations that can be made using the data in the Fig.2 Strain capacity is determined from these tests under rapid and slow loading to simulate both rapid and slow temperature changes in the concrete.3 shows the unit strain values 7.4 Time. However.521+0 0 7 0 0 LOG. (60 x 60 cm) in cross section and 64 to 130 in. The strain can also be determined from deflection measurements.5.2 and Table 7.2 . but the rate of creep increases.2B).66 x 5. 7. 7.14 Fig.231+0. Autogenous volume 7. Expansion can be helpful in preventing cracks.6 psi/F (0.1.0 to apply to tension as well.5. Detailed test procedures can be found in References 7. to failure-at about 3 months.CONTROL OF CRACKING 224R-31 thus. 7. 7. (28 x 56 cm) size and with 3 in. volume change7.4 Modulus of elasticity .7. (1+1 ) -9OCOyS 1 Day 1 30 “E Y 2 20 .1’ gives useful coefficients for converting creep of smaller aggregate concrete to creep for mass concrete. respectively. (1+1) -3Oayr E =0384+0. creep as measured in compression is assumed 2. 1’ 1. Again.8 2. and if fully restrained would be stressed 0.0500 Log ( 1 + 1 ) b -28asDays E =0. it is a selfinduced expansion or contraction.0294 LOG. Relatively large beams ranging from 12 x 12 in. and continued periodically for months. The symposium on creep of concrete.2 shows typical creep data obtained from laboratory investigations.Typical concrete creep curves for mass concrete. T o t a l Strain E=l481+00547~ (I+11 ---Icq c =0. The measurement is usually made by means of embedded strain meters. When the stress exceeds about 60 percent of the ultimate and microcracking occurs. Thus. change is usually measured by strain meters embedded in concrete cylinders which are carefully sealed (to insure that there is no loss in moisture) and kept at constant temperature.2.1” Table 7.) of l/1.7 For long-term tests. Testing should be conducted at a constant temperature for maximum accuracy in measurement.2A).5 psi per F = 3.2 0 I” 0. Creep of concrete is measured on carefully sealed specimens stored at a constant temperature and loaded to a constant stress. 38 i ’ lb/in.76 1. however.1 and 7. Post-cooling of concrete is accomplished by circulating cool liquids (usually water) through pipes embedded in the concrete.08 __ of lineal thermal expansion of concrete assumed to be 5.33 1.46 0. Sustained modulus Es at age of concrete at time of loading. A vulnerable location in pipe cooling systems is centered at the cooling coils where sharp gradients and cracking can be induced if termination of cooling water circulation is not timely. It is._~ values are based on data given in E.64 0.1* 7.35 2 kg /cm x 10-3 psi x 10-6 b 7 days I t I !1 x 1 0 -3 183 172 151 139 134 123 113 95 t 2.---. Resistance thermometers should be used in sufficient numbers to permit adequate monitoring and control of the internal concrete temperatures.Artificial cooling by embedded pipe systems Construction drawings should show basic pipe layThe overall program for cooling concrete.7 8.44 ’ .94 I I ! / I 3.z ______ ______ ~. j 1. 7.20 may be used. and the ing important field control criteria.12 0.15 1.Illustration of computation of sustained modulus of elasticity (Es) and stress coefficients A.66 0. and approximate duration of cooling. .9 7.99 I t-- x 10 -3 ------ I ’ I- / 303 263 234 210 .Unit tensile strain versus beam stress (References 7. partial. 7.5 m TGkg.7).em’)C ’ 11.4. transverse construction joints. A pipe layout for a typical concrete methods..9 millionths/C. 7. important that steep cooling gradients. includout and spacing including minimum spacing. days 1 day Time after loading days psi -6 x 10 0. the duration of cooling and the heat removed by the pipe cooling should be sufficient to insure that a secondary internal temperature rise in the mass does not exceed the primary rise. versus beam stress at outer fibers for a typical laboratory investigation._______-___. Studies made during the design stage will establish such items as lift height..8 44.1 psi -6 x 10 1.6 .45 0. This is particularly true in smaller masses where circulation of cooling water should be stopped when the maximum temperature has been reached and just begins to drop.47 0.70 1.7. mined during the design stage. and using small ice particles as a replacement of part of the mixing water.60 2. should be deterlayout at dam faces.-.2 .4 1 . pipe spacing.10-3 47.2 44. TABLE 7.62 1.5 3.81 1.2 3 days 2 kg/cm .46 2. approximate methods for estimating tensile strain capacity under rapid and slow loadings given in References 7. Tensile stress coefficients for condition of full restraint and decreasing temperature Age of concrete at time of loading 1 day 3 days kg/cm’/C 7 days Ib/in.50 1. Precooling concrete interior openings and in sloping.7 3. 7.11 In the preliminary studies of temperature and construction control plans for mass concrete projects.___ unit elastic strain/psi + Vz specific creep for time of loading R.68 0. which can result in cracking the mass.31 2.‘/F 14 14 12 11 I kglcmJ/C 1.76 3. acceptable rate of temperature drop (for both rapid and slow drops). In general. be avoided.33 3. including cooling all ingredients of the mix lift is shown in Fig.5 and 7.00 2.63 (1) Sustained modulus of elasticity IE Fig.98 t I I x 10 -6 4.61 2. i I I 0.34 2.22 1.224R-32 ACI COMMITTEE REPORT B E A M STRESS OF OUTER FIBERS I Fig.:” i ~~g!~~/~ 24 21 18 16 .0 9.lb/in.5 mil lionths/F (9. and isolated prior to placement is accomplished by a variety of concrete lifts.6 46.‘/F 0 1 3 7 (2) Coefficient 3. water temperature and rate of flow.6 3.3 .92 1. 7.Schematic of embedded pipe cooling embedment system in mass concrete. it must be decided if cooling pipes to isolated areas in the foundation and at openings such as galleries can extend from the downstream face of the dam or if a vertical riser must be used.11). but isolated areas always exist in all dams which tend to result in a concentration of pipes. In this case.Elev. the pipe used for postcooling should be thin wall tubing.4 .CONTROL OF CRACKING 224R-33 8 Multiply 4@2'-O" By 0 0254 0 3048 To Obtain Meters Meters 8 s 3 w . For ease of installation. Compression type couplings are used because thin wall tubing cannot be threaded satisfactorily. 7. steel tubing is preferred. In most areas of the dam. Surface connections to the cooling pipe should be removable to a depth of 4 to 6 in. 1140 K-1 Elev 1135 Detail " B " Section A-A Fig. Aluminum tubing is lightweight and easy to handle. breakdown from corrosion inducing elements of the concrete is a potential problem for aluminum pipe if cooling activities must be carried on over a period of several months. Fig. and at isolated and sloping lifts of concrete. For example. Also. These concentrations tend to occur at the downstream face of the dam where inlets and outlets to cooling pipes are located. However.-&--. it must be determined to what extent the cost saving procedure of concentrating cooling pipe inlets and outlets near contraction joints can be permitted at the face of the dam. (102 to 152 mm) so . Proper planning will alleviate many of the undesirable conditions that can result from these concentrations. I I35 COIL LIN FEET 1" = 30'-0" +_FlDw 47 r-.Typical cooling coil layout (Reference 7.5 . adjacent to openings in the dam. Inches Feet W ~_ W PLAN ELEV. a uniform spacing can be maintained for the cooling pipe. Many factors which affect the . Also. is subject to a number of additional factors which.7.Summary . it should be thoroughly flushed with water at a high enough pressure to remove foreign matter and grouted full with a grout mixture compensated for plastic shrinkage or settlement. low cement content.7. Sight flow indicators should be installed at the end of each embedded pipe coil to permit ready observance of cooling water flow. It is not necessary to use a mortar layer on lift surfaces prior to the placement of the next lift. and pleasing in appearance. In addition to regular observance of flows. Sufficient standby components. The strength is usually derived from tests on cylindrical specimens which are not completely representative of the structure.S. For specific data on appropriate reduction factors.2 Safety against sliding. For such reasons.1 Safety against crushing-concrete strength. Burau of Reclamation. 7 2 Economy . Therefore. This. in turn. There can be a large gain after 90 days depending upon the composition of the cement. For interior concrete. 7. The 90-day strength is often used and is derived from tests of job cylinders. even a “factor of safety” of three is far more than enough to cover any likely differences between plus and minus corrections.7 . durable. the lift surfaces should slope slightly upward toward the downstream edge (in the case of a dam) such that the downstream edge is higher than the upstream edge. However. 7. absence of leakage and leaching. Forms should be designed and constructed so that shutdown of cooling activities is not necessary when forms are raised. However. the lowest practical strength should be specified so as to reduce the cement content.7.80 for typical conditions. water temperatures and pressures and concrete temperatures should be observed and recorded at least once daily while the lift is being cooled. The refrigeration plant for cooling water may be centrally located. or several smaller complete portable plants may be used to permit moving the refrigeration system as the dam progresses upward. hardened horizontal lift joints may impair the safety. Durability will depend upon the quality of the concrete. However. 7 . 8th Edition. 7. The importance of a comprehensive materials test program to establish nec- essary control prior to preparation of construction controls and specifications cannot be overemphasized. 7. Concrete Manual.1. etc. meaning that the strength should be three or four times the expected maximum stress. Coils must be pressure tested for leaks at the maximum pressure they will receive from the cooling system prior to placing concrete. will reduce the heat of hydration and the consequent thermal stresses. should it occur. balance one another. After cooling is completed and the pipe is no longer needed. 7. the specifications should require care in the preparation of lift surfaces and in the placement and compaction of concrete thereon. Since the cylinders are made from wet screened concrete.Basic considerations for construction controls and specifications The construction controls and specifications for mass concrete must be such that the structures will be safe. thus increasing the crack resistance of the concrete. The strength at 90 days is not the ultimate strength. exposure conditions. uncracked concrete provides a very large factor of safety against sliding. more or less. The grout should remain under pressure until final set is attained. the reader should refer to the U.” as defined above.5 shows the schematic layout of a typical pipe cooling system. neither the strength nor the maximum stress can be accurately determined.1. The “factor of safety. and freedom from chemical reactions of a deteriorating nature. Wire tiedowns embedded at the top of the concrete lift at about 10 ft (3 m) spacing satisfactorily secure the pipe during concrete placing. adequate but not excessive temperature control. the measured strength is corrected to a massconcrete equivalent by applying a reduction factor of about 0. economical. freedom from cracks and stains. Each of these requirements in turn affects the crack resistance. Pleasing appearance will come from good workmanship. More than the necessary amount of cement is detrimental rather than advantageous. A strength should be specified which will provide an adequate factor of safety against crushing of the concrete.224R-34 ACI COMMITTEE REPORT that holes can be reamed and dry packed when connections are removed. Since the average strength of the job cylinders is used. equal in capacity to the largest individual refrigeration units should be provided. Safety will be assured if the concrete has sufficient strength and continuity (absence of cracks). half of the tests will be weaker. Economy will depend upon such features as the best choice of aggregates. The maximum stress is usually taken as the design stress which is based upon assumed concrete properties. etc. Sound. Pressure must also be maintained during concrete placement to prevent crushing and permit early detection of damage.1 Safety 7. Fig. The “nominal” factor of safety is merely the compressive strength divided by the maximum stress to be expected in the structure. it is considered good practice to use a safety factor as high as three or four. it reduces the expansion due to reactive aggregates when such are encountered.” ACI JO U R N A L. This can result in a considerable saving in cost. probably 2 ft (0. 1967.1 Selection of aggregate. Donald L. 7. by using a water-reducing agent when appropriate.. S.6 m) or less. Mar.5.S. No..6 Liu. and it should produce particles of good shape and surface texture. Air entrainment should be mandatory. E. 1968.. except for concrete which must resist high-velocity water flow. Jr. . 67. good pozzolans such as fly ash are available. Department of Civil Engineering. Houk. James A. Berkeley. 7.40 by weight. and Bombich. The effect of the rich boundary concrete on thermally induced cracking will be minimized by keeping the thickness of the boundary layer to a minimum. and Wilson. Proceedings V .3 Durability . p p . A. Larger aggregate permits the use of less water and cement per cubic yard.. References 7. University of California. 73.. 7. Tony C. 7. therefore. Aggregate should be chosen that makes good concrete with the lowest overall cost. crushing to increase crack resistance may be an economical expedient because of the consequent saving in temperature control. for example. Dec. A reduction in the water content of concrete permits a corresponding reduction in the cement content.2. Dec. 1979. Apr. Sandhu. 7. Some of the factors which affect economy are discussed below. 7. the water-cement ratio of surface concrete should be kept lower than that necessary for strength alone.8. When crushing is either advantageous or necessary.” Miscellaneous Paper No. a low modulus of elasticity. With proper planning and execution. For example. Wilson. No. Vicksburg.” ACI J O U R N A L . 153-166. 7. M..CONTROL OF CRACKING 224R-35 economy also affect crack resistance. it can reduce the heat generation and improve the resistance against cracking. E. less drying shrinkage.S. The aggregate which makes concrete of highest tensile-strain capacity may increase the water requirement and. and they can be used to replace a portion of the cement.3.4. Structural Engineering Laboratory. 1969. and as a result is more durable and crack resistant. A. pp. the procedures presented will serve as useful tools in developing a crack control program for mass concrete structures. As indicated in Section 7.Durability of concrete is closely related to the exposure conditions. 7. Army Engineer Waterways Experiment Station. U. “Verification of Temperature and Thermal Stress Analysis Computer Programs for Mass Concrete Structures. there may be no deteriorating influences acting on the concrete except that which is subject to high-velocity water flow. Ivan E. “Finite Element Analysis of Nonlinear Heat Transfer Problems. R... any concrete which has the required strength can be expected to last indefinitely. thus offsetting the benefits of high strain capacity.” Report No.4. “Prediction of Thermal Stress and Strain Capacity of Concrete by Tests on Small Beams. the least expensive aggregate may have bad thermal properties and thus require expensive temperature control to prevent cracking.” Proceedings. 3 pp. For any concrete which might be subject to both alternations of freezing and water pressure. P02. ASCE. Army Corps of Engineers. 3. “Method of Test for Thermal Diffusivity of Mass Concrete. 1973.. Dec.. Where the climate is severe.. U.2 Aggregate size.. “Concrete Volume Change for Dworshak Dam.” Report No. The largest maximum size of aggregate..” (CRD-C 37-73).2. minimum water content can be achieved by specifying adequately powerful vibrators which permit the use of low slump concrete.7. “Determining Tensile Strain Capacity of Mass Concrete. such that there is much freezing and thawing in winter.2. and the cement content should be kept low to minimize heat generation and resultant potential cracking. The appropriate amount of pozzolan for a reactive aggregate should be based upon test data obtained with the pozzolan and cement being used.2. Paxton.7. and possibly more important.4 Control of cracking .7. rock which has the most favorable properties should be chosen. L.7.4 Use of pozzolan In most locations. Proceedings V. J. L. Berkeley. 1970. resulting in savings in both the amount of cement and the amount of temperature control necessary for required crack resistance. L.7.7. and by producing and placing the concrete at low temperature. and Hough- ton. L. The rock should have a low coefficient of thermal expansion. In tropical cli- mates. M. Campbell. 12. “Two-Dimensional Stress Analysis with Incremental Construction and Creep. 95. “The Determination of Temperatures within Mass Concrete Structures. pp. 7.A detailed discussion of the control of cracking in massive structures has been presented in this chapter. The concrete with less water and cement is superior in many ways: it undergoes less temperature change.. June 1976. 7. Donald L. Dec. also the cement requirement. 253-261. UC SESM 76-2. Houghton.7. Houghton. 1976. Berkeley. 67-34.1. Oct. should be specified as can be placed properly in the structure. 7. and Raphael. 7. All of these factors are important in increasing the resistance of the concrete to cracking. R. If natural aggregate near the site has unfavorable properties for crack prevention. E. Vicksburg.2.. For the main structure in such a case. University of California. the water-cement ratio should be less than 0. Structural Engineering Laboratory. Another advantage of using pozzolan is that when used in adequate amounts. (150 mm) in diameter. Wilson. University of California. Handbook for Concrete and Cement. SL-79-7. Donald L. 7. V. up to approximately 6 in. R. 691-700.3 Water content. Polivka.” Report No. 68-17. and McDonald.A wall or parapet anchored along its base to the foundation or to lower structural elements less subject or responsive to volume change. “Studies of Autogenous Volume Change in Concrete for Dworshak Dam. Trumbull Pond Dam. there is a minimum of cracking.S. early age but which might be sustained at greater maturity. U. B. McDonald. specifications. E.11. 7. concrete should have a high tensile strain-to-failure capacity.5 . as used in this chapter. differential strains develop and tensile stresses are induced. Cracking is usually inevitable unless contraction joints (or at least grooves of a depth not less than 10% of the wall thickness on both sides. 8.Restraint of flat work results from anchorage of slab reinforcement in perimeter slabs or footings. 627 pp..2. and tensile strains develop which may cause the exterior to crack. McCoy. Vicksburg.. and a low modulus of elasticity in tension is desirable.Wall. SP-9. Ivan E. 1975. it is worthwhile to mention the basic cause of cracking. 8. Army Engineer Waterways Experiment Station. temperature reinforcement can restrain the shrinkage of surface concrete. Dec. No.3 .S..Introduction Construction practices. 5 pp. materials. in which the cracks will occur and be hidden) are provided at intervals ranging from one (for high walls) to three (for low walls) times the height of the wall. 8. Dworshak (Bruce’s Eddy) Dam. Donald. the interior concrete restrains the exterior concrete from shrinking.2 . Handbook for Concrete and Cement.” (CRD-C 39-55). R. This points to the importance of protecting new concrete for as long as practicable from the loss of moisture or a drop in temperature. slabs. Detroit. 8. since the effect of temperature change or drying shrinkage will be different in the two sections.” Miscellaneous Paper C-72-20. Jr. Liu. J. pp. E.6 . and mix considerations.Restraint Restraint exists in many circumstances under which the structure and its concrete elements must perform. 6-613. K. This occurs when the surface cools. 192-197. 2 pp. No. As noted earlier. 1964.21 . T. Handbook for Concrete and Cement. if the concrete is not strong enough to withstand the tensile stresses developed.” ACI JOURNAL .S.12. Strain Gage Method. Vicksburg. A. and tunnel linings placed against the irregular surface of a rock excavation are restrained from moving when the surface expands or contracts in response to changes in temperature or Chapter 8 . Tuthill. When a slab is free to shrink from all sides toward its center.16. June 1963. Preferably. When this happens. Army Corps of Engineers. U. When these differential responses exceed the capability of the concrete to withstand them at that time. Before discussing control of construction practices which affect cracking. July 1969. particularly in heavier sections. Borge. 66. “Ultimate Strain Capacity and Temperature Rise Studies. North Fork Clearwater River Near Orofino.2. and inherently. 7. while the interior is still warm from the heat of hydration. Symposium on Creep of Concrete. Army Engineer Waterways Experiment Station.. particularly the latter. Vicksburg. Contraction joints and perimeter supports should be designed accordingly (see Section 3. Obviously. 8th Edition. 8. cannot be free to respond to the same degree to volume changes. Denver. Idaho: Creek Tests..Restraint will occur at sharp changes in section. 7. A. 8. all parts of concrete structures are not free. a contraction joint can be used to relieve the restraint. or when the surface concrete dries faster than the interior concrete...2. 8. Proceedings V. Bureau of Reclamation.” ACI JOURNAL . U. Aug. “Concrete Laboratory Studies.224R-36 ACI COMMITTEE REPORT 7. pp.10. Jr. Consequently.1 .2.2. 7. “Method of Test for Coefficient of Linear Thermal Expansion of Concrete. It is restraint. 7. “Method of Test for Coefficient of Linear Thermal Expansion of Coarse Aggregate. Concrete Manual. and Houghton. Proceedings V. 1972. and Sullivan. May 1978. “Prediction of Tensile Strain Capacity of Mass Concrete.13.S. 7. These considerations may result in stresses capable of causing cracks at an *Principal author: Lewis H. Vicksburg.14. Houk. . Typical examples will illustrate how restraint will cause cracking. If all parts of the concrete in a concrete structure are free to move as concrete expands or contracts. American Concrete Institute. Bombich. it is often feasible to protect the surface for a time at early ages so that such stress-inducing differentials cannot develop before the concrete is strong enough to withstand the strain without cracking.” Miscellaneous Paper No. will change temperature or moisture content at different rates and to different degrees.2 .Acting similarly to the interior concrete in the foregoing example. Army Corps of Engineers. 5. 560-568. 75. This is influenced greatly by the aggregate.Control of cracking by correct construction practices* 8. James E. Report 2.. however.. 1939. Thorton. H. If feasible. but more and narrower cracks may result. as well as on-the-job construction performance.15. and Allgood.5. will be restrained from shrinking when its upper portions shorten due to drying or cooling. 7.S. there will be no cracking due to volume change.9.Exterior and interior concrete. cracking occurs.. 1964.” (CRD-C 125-63). Orville E. U.. include designs.3). E.4 . U. 7. 160 pp.. J. Tony C. 1. Where surface drying may be rapid.2. how dry the surface concrete becomes. which is a key contributor to the formation of cracks in concrete.The typical examples presented above clearly indicate that many crack control procedures must be considered by the engineer during design. per se. As the moisture is removed. concrete loses moisture slowly.1. 3. It will cause shrinkage strains of up to 600 millionths or more. Cold concrete (below 50 F. Accordingly.2. as shown in Fig. The importance of this will vary with the weather and the time of year. as mentioned in connection with tunnel linings and conduits. and the use of cold concrete (50 F or 10 C) has reduced cracking materially. not cement content. In tunnel linings.5. the shrinkage in the first few weeks is primarily thermal. the surface concrete contracts. 3. and 4.1 this can be done if a bulkhead is used to prevent air movement through the tunnel. Of major importance is the selection of mix proportions that require the least amount of water per cubic yard for the desired concrete strength. how much mixing water was in the concrete. the coarser the sand should be and the less there should be of it in the mix). more care must be devoted to uninterrupted curing to get good surface strength. so that it will have time to develop more strength to resist cracking forces. provided the relative humidity is above 40 percent. resulting in tensile stresses in the essentially strengthless. that cause short random cracks or openings in the sur- . in accordance with correct principles of concrete proportioning.Plastic shrinkage cracks occur most commonly. as well as in Reference 8.3 . The amount of shrinkage cracking depends on 1.2.3.2). the wet curing cover can be allowed to remain several days without wetting after the specified curing period (preferably 7 to 10 days).CONTROL OF CRACKING 224R-37 moisture content. Contrary to common belief. Typically.3. If the tunnel carries water. Moreover. they should be required in the specifications for the work. in the surfaces of floors and slabs when the ambient job conditions are so arid that moisture is removed from the concrete surface faster than it is replaced by bleed water from below. 3. If job conditions are likely to be such that these measures will be worthwhile. the contractor cannot be expected to utilize the best procedures. This is because the water requirement of concrete does not change much with a change in cement content. until the cover and the concrete under it appear to be dry. circumferential cracks in tunnel linings and other cast-in-place concrete conduits and pipe lines can be greatly reduced in number and width. 8. The use of the lowest practical slump is important.Shrinkage The following sections discuss the major causes of shrinkage. 8. and using well-graded sand with a minimum of fines passing the l00-mesh and free of clay. using aggregate with the most favorable shape and grading conducive to best workability. As shown in the Bureau of Reclamation Concrete Manual. the extensibility of the concrete.3 Plastic shrinkage .7 .1 Effect of water content . Concretes having a low strain capacity are much more sensitive to shrinkage due to drying (and to drop in temperature) and will be subject to a greater amount of cracking. 8. the concrete lining is much stronger and better able to resist shrinkage cracking. This means avoiding oversanded mixes (the richer the concrete. Drying shrinkage is proportional to water content (Fig. At some depth. The latter is largely related to the composition of the aggregate and may vary widely.8. Such a hypothesis is clearly indicated in Fig. However. will offset any tendency to increase the water requirement. The extensibility represents how much the concrete can be strained (stretched). If it does not. and objectionably. To accomplish this.The greater the water content of concrete. does not necessarily cause an increase in shrinkage. 10 C) dries very slowly. using the largest maximum aggregate size practical. closely spaced contraction joints or deep grooves must be provided to prevent or hide the cracks which often disfigure such surfaces. a prime objective of crack control procedures is to keep the concrete wet as long as feasible.2 Surface drying . the reduction of the amount of fine aggregate to compensate for the added cement. These cracks occur prior to final finishing and commencement of the curing process. some concretes of highly quartzitic gravels have a low strain capacity and a high modulus of elasticity. without exceeding its tensile strength and is the sum of creep plus elastic strain capacity. By the time drying is significant. As discussed in Section 8. 2. the character and degree of restraint involved. unless these procedures are included in the designs and specifications on which the bid price is based. if the surface is prevented from drying quickly at the end of the curing period. 3. and shallow ponds of water are placed in the invert as soon as possible after lining. increasing the cement content of concrete. while some concretes of granitic and gneissic aggregate have a high strain capacity and a low modulus of elasticity. the more it will shrink on drying. such that its sand equivalent value is not less than 80 percent AASHTO T176.3. stiffening plastic concrete. 8. Cracking stresses will be further reduced by creep. the concrete will have become much stronger in the humid environment and will be better able to resist shrinkage-induced tensile stresses.Surface drying will ultimately occur except when the surface is submerged or backfilled. 8. While proper construction performance can contribute a great deal (as will be discussed below). there will be no further drying shrinkage. and left until the tunnel goes into service. form bolts or other embedments. The amount of shrinkage is reduced by restraint and creep. a vigorous effort should be made to close the cracks by tamping or beating with a float. is a function of the aggregate and should be evaluated. A check will appear at these locations. or in many cases to eliminate it. What applies to one will not necessarily apply to another. if the forms are hot at the top or are partially absorbent. At the first appearance of cracking while the concrete is still responsive. low slump. Cracks often appear in horizontal construction joints and in bridge deck slabs over reinforcing or form bolts with only a few inches cover.2). If firmly closed. pozzolans for a portion of the cement. curing should be started at the earliest possible time. The earlier the age and the slower the rate at which cooling or drying occur. water reducing admixtures. are 1. This is due to the relaxing influence of creep. it is advisable to postpone these operations as long as possible to get their maximum benefit without the recurrence of cracking. In other cases. 2. But. See Fig. In addition to Chapter 7 of this report. Plastic sheeting can be rolled on and off before and after floating.5 . These cracks are usually rather wide at the surface but only a few inches in depth. Least effective but helpful are certain sprayed monomolecular films which inhibit evaporation.1 of ACI 207. and as such.8.224R-38 ACI COMMITTEE REPORT face. 8.2 Properly executed late revibration can be used to close settlement cracks and improve the quality and appearance of the concrete in the upper portion of such placements. In ordinary concrete work. 3. even though settlement has taken place and slump has been lost.4 and Fig. it is desirable to schedule flatwork after the walls are up (ACI 305R.The aggregate should be one which makes concrete of high strain capacity. Conditions most likely to cause plastic shrinkage cracking are high temperatures and dry winds. Such actions include the following: 8. The cracks generally range from a few inches to a few feet in length and are a few inches to two feet apart.2. using a well dampened sub-grade. and 3. The cracks in bridge decks can be reduced by increasing the concrete cover. large aggregate. The system of contraction joints and grooves previously discussed for control of shrinkage cracking will serve the same purpose against substantial later drops in surface temperature.7.1 Concrete aggregates . 6. no more cement than necessary.Surface cooling will shrink the surface of average unrestrained concrete about 10 millionths for each deg C (5.4 . 3. using cold mixing water or chipped ice as mixing water to lower the temperature of the fresh concrete. Sometimes plastic shrinkage cracks appear early enough to be worked out in later floating or first trowelling operations. They are not due to any of the causes discussed above. especially on larger projects.2R. last but by no means least. the lower the tensile stresses will be. Primarily. the winter protection required for the development of adequate strength will prevent the most critical effects of cooling. chipped ice for mixing water to reduce the temperature of the fresh concrete as much as possible. they may reappear if they are merely trowelled over.3. but tensile stresses are induced.1R discuss temperature controls for mass concrete to minimize the early temperature differences between interior and exterior concrete. as noted in Section 8. these controls lower the interior temperature rise caused by the heat of hydration by using 1.4 Surface cooling . When this is successful. specifications should stipulate that effective moisture control precautions should be taken to prevent a serious loss of surface moisture under such conditions. Other helpful practices that may augment the bleeding and counteract the excessive loss of surface moisture. ACI 302. air-entrainment. if reasonably available (see Section 7. Accordingly. At no time should forms be removed to expose warm surfaces to low temperatures. the extensibility.1R). However. Fine and . 5. Chapters 4 and 5 of ACI 207.Construction A great deal can be done during construction to minimize cracking.2. and 7. after the initial vibration.3. 8. an earlier than normal floating may destroy the growing tension by reworking the surface mortar and prevent plastic cracking that would otherwise occur. As mentioned in Section 8. 8. in 100 ft with a drop of 50 F). which imparts more extensibility to concrete at early ages.5 millionths per deg F) the temperature goes down. such actions must be required by the specifications and by the engineering forces which administer them. they will be monolithic and are unlikely to reappear. Principal among these precautions are the use of fog (not spray) nozzles to maintain a sheen of moisture on the surface between the finishing operations. but are the natural result of heavy solids settling in a liquid medium. Windbreaks are desirable. This amounts to 9 mm in a 30 m length with a drop of 30 C (l/3 in. In any event.5. cooling the aggregates by dampening and shading them. preferably exposing only the area being worked on at that time. or strain the concrete will withstand before tensile failure.Settlement Settlement or subsidence cracks develop while concrete is in the plastic stage. where at all practicable. 4. Sometimes concrete will tend to adhere to the forms. 2. Settlement cracks occur opposite rigidly supported horizontal reinforcement. 3. when com- of sand or fly ash. is always popular and in demand on the job. or even higher air content (which may reduce strength). and this should be verified frequently (AASHTO T176). others in addition. 8. lead to demands for a greater margin of workability. invariably. 8.Expansive cement can be used to delay shrinkage during the setting of concrete in restrained elements reinforced with the minimum shrinkage steel required by ACI 318. it contains a large amount of adsorbed air. in addition to opposing settlement during hardening. as opposed to grouts that expand only in the plastic state and later suffer drying shrinkage. The hydrogen gas tends to expand the mixture and thus prevents subsidence and may even cause expansion. One commercial grout is so highly acceler- .6 Cold concrete . Among the admixtures that merely prevent settlement. It must be discouraged if the best concrete for the work (having adequate workability with proper handling and vibration. mortars and concretes employing these agents have no expansive properties after hardening. such as exists in portland cement mixtures. These. With correct usage (particularly with early and ample water curing on which maximum expansion depends). The sand should have a sand equivalent value in excess of 80 percent. salt spray.5. This is best done by finish screening and rinsing as a combination of coarse aggregate sizes goes to the batch plant bins. overlap of sizes. Grouts which expand (if unconfined) after hardening can function as nonshrink grouts. T h e principal property of these cements is that the expansion induced in the concrete while setting and hardening is designed to offset the normal drying shrinkage.5.5 Excessive workability . segregation. 8. This acts to hold the grout tightly up under base plates. Details on the types and correct usage of shrinkage compensating cements are given in ACI 223-83. 8.005 to 0. and also tends to offset the effect of drying shrinkage. 8.Cold concrete. This settlement can be objectionable if a space is to be filled up tightly without leaving a void at the top. These should contain no stearates. Still another contains a form of carbon with a very large surface area.Ordinarily. the distance between joints can sometimes be tripled without increasing the level of shrinkage cracking. s o t h a t e x t r a amounts of fines are not needed in the mixes to account for variations in grading without a serious loss of workability. Therefore.5. Another is composed of organic gelling compounds of soluble cellulose which increase in viscosity so that the solid particles remain in suspension. and concrete mixtures will settle before hardening.5. particularly clays. and breakage. the aluminum reacts to form aluminum oxide and hydrogen. and have a drying shrinkage at least equal to similar plain grouts. such as under machine bases. In the dry form. and the richness of the mix..Whether it is achieved with unneeded higher slump. it is advisable to make trial mixes with various percentages of aluminum powder to find which percentage gives the desired (slight) expansion under the prevailing conditions. It is not possible to specify an exact percentage because the amount to be used varies with such factors as temperature. are subject to shrinkage if exposed to drying and may deteriorate and lose serviceability if exposed to an aggressive environment (weathering. there is one containing a metallic aggregate which. Among the commercial admixtures. Where feasible.CONTROL OF CRACKING 224R-39 coarse aggregates have to be clean and free of unnecessary fine material. Gas forming agents and air releasing agents produce the same net effect. like any portland cement grouts and mortars. This diluted mixture will have enough bulk so that it can be easily measured and properly dispersed in the mix. with more sand and more water in the concrete. The amount of aluminum powder used is so small that it is advisable to dilute it by blending with 50 parts ated that it starts setting before settlement takes place. or concrete. etc. or fatty acids. mortar. It should be noted that prepackaged “Non-shrink” grouts. The most widely used materials contain unpolished aluminum powder.01 percent by weight of the cement. mortars and concretes not employing them. The sand should have sufficient time in storage for the moisture content to stabilize at a level of less than 7 percent on an oven-dry basis.Should be done with all practical care to avoid contamination.4 Handling and batching . mortar. Every effort should be made to uniformly batch and mix the concrete so that there will be a minimum of troublesome variation in slump and workability. and water will rise. instead of a fluid grout or mortar. but is usually in the neighborhood of 0. provides a modest expansion after hardening. oversanding. and having minimum shrinkage factors) is to be obtained. alkali content of the cement. Measures taken to prevent such subsidence have produced what is known in the trade as “Non-shrink” grout. Grout mixed in a colloid mill will not readily settle. Some of the materials merely prevent settlement. which is released gradually into the mix producing an expansion. although all grouts. or concrete . some of it to the top surface.3 “Non-shrink” grout. a number of different mechanisms are in operation.2 Expansive cement . The amount of aluminum powder used varies widely with conditions. palmitates. mortar. In an alkaline solution.5. the solids in grout.). etc. the problem of settlement can be solved by the use of dry tamped mortar. small aggregate. provide a slight expansion as the mixture hardens. Cement should be Types I. In cold weather. 8.Newly placed concrete must be brought to a level of strength maturity and protected from low temperatures and drying conditions which would otherwise cause cracking. 8.1. particularly if use of calcium chloride is permitted.5. Subgrade and other supports must not settle unevenly. Aggregates favorable to low mixing water content are (a) well graded. The following items should be carefully spelled out in the specifications. Finishing should not be done in the presence of surface water. Cold concrete is particularly useful for massive concretes. any reactive elements of aggregate should be neutralized through the use of low alkali cement or a suitable pozzolan. Control joints. 4. It requires less mixing water and thus reduces drying shrinkage. It is pointless to expect to protect surfaces.2). 10.5. and corners by placing needlessly warm concrete in cold weather. or preferably both. mixes. Certain cherts and other expansive aggregates and lignite can cause cracks at popouts. Deep revibration corrects cracks caused by differential settlement around blockout and window forms. Curing should be prompt. concrete is naturally cold and every effort should be made to use it as cold as possible without inviting damage from freezing. will eliminate cracks and checks where something rigidly fixed in the placement prevents a part of the concrete from settling with the rest of it. In warm weather it expedites the work by reducing slump loss. These vulnerable parts must be protected with insulation or protective enclosures (ACI 306R). and the wet cover should be allowed to dry before it is removed. V. expansion and cracking. Aggregate should consist of rock types which will produce low-shrinkage concrete (see Section 3. Low-slump concrete should be used. or IS. 8. If it is used. edges. and (c) free of clay. 3. 8. to prevent cracks due to overstress in the structure. Special care is needed in handling precast units to prevent overstress due to handling. calcium chloride must be limited to the absolute minimum (see Section 3.224R-40 ACI COMMITTEE REPORT bined with factors to reduce water and cement content to a practical minimum will reduce temperature differentials which cause cracking.5.4). (76 mm) slump is rarely necessary except perhaps in very hot weather in which both slump and moisture are lost quite rapidly. 9. discussed in Sections 3. 4. Settlement cracks are most apparent in the upper part of wall and column placements where revibration can be readily used.10 Miscellaneous .3. 2. flat. 8.Some items normally covered in specifications (or certainly which should be covered where appropriate) require special attention during construction because of their potential effects on cracking. the contractor cannot be expected to provide other than ordinary materials. II.Flatwork finishing can make a great difference in the degree of freedom from all types of cracking (ACI 302. Concrete should not be placed against hot reinforcement or forms. Reinforcement and embedments must be properly positioned with the designated thickness of cover in order to prevent corrosion.Specifications to minimize drying shrinkage Actions during construction to obtain the lowest possible drying shrinkage must be supported by the specifications. Formwork support should be strong enough to be free of early failures and distortion causing cracking. It is obtained by substituting chipped ice for all or a part of the batched mixing water. 3.1R). 8.7 Revibration .9 Curing and protection . of full duration.2. Unvented salamanders in cold weather (ACI 306R) or gasoline operated equipment must be avoided where adequate ventilation is not furnished. increasing pumpability.5.4) should be taken to prevent plastic shrinkage.3 and 8. stipulated in Section 8. creep will have an opportunity to reduce the possibility of cracking when the curing and protection are fully discontinued. Correct amounts of entrained air should be specified and used to prevent cracking due to freezing and thawing and exposure to calcium or sodium chloride. More than a 3 in. Job specifications should cover these aggregate properties and constructors should ensure observance of these requirements. 1. or splintery). Unless bids are taken on this basis. Precautions (see Section 8.5.6 .6.4. must not be omitted and grooves must be of the specified depth and well within the maximum permitted spacing.4. and by improving the response to vibration. 6. and procedures. dirt.When done as late as the formed concrete will respond to the vibrator. 8. 1. 7. (b) well shaped (not elongated. If the new concrete is given a few days to gradually dry or cool. 5.6. The curing and protection should not be discontinued abruptly.They can have an important influence on drying shrinkage. In addition to cleanliness of aggregate. 2. . and excess fines.8 Finishing . Any required marking and grooving should be carefully cut to the f u l l depth specified. because of the danger of carbonation shrinkage surface cracking. Calcium chloride should be prohibited.1 Concrete materials .3. preferably not Type III. and where slabs and walls are placed monolithically. Contact between aluminum and steel embedded in the concrete must be eliminated. 8. the engineer. but not less than 1 in.CONTROL OF CRACKING 224R-41 8.6.1. 5. special emphasis must be placed on obtaining effective locations and an adequate number of contraction joints. Water curing should use a wet cover in contact with the concrete surfaces. a properly applied sealing compound provides good curing for flatwork placed on a well-wetted subgrade and provides adequate curing for massive sections. the cover should be left in place until it and the concrete surface appear to be dry. retaining. The largest practical maximum size of aggregate (MSA). more fines. Exposure of warm concrete surfaces to fast drying conditions or to low temperatures prior to curing.Finishing should follow the avoid all forms of surface cracking. recommendations of ACI 302. 4 to 3/8 or 3/4 in..2.Plans should include an adequate system of contraction joints to provide for shrinkage. 8. more water. and other walls at the depth and spacing indicated in Sec. The lowest practical temperature. Bureau of Reclamation. belts. In less arid areas and for interiors. and Carrier. the forms should be left on with loosened bolts long enough to allow the concrete surfaces to dry gradually. In an arid climate.3 Concrete handling and placing . The lowest practical sand content. (25.6. U. especially in arid weather.75 mm to 9. 4. in order to expedite pumping. Concrete M a n u a l 8th Edition. the use of pumping equipment capable of handling mixes favorable to least cracking should be required.For least shrinkage. 8. To assure both the owner’s and the engineer’s satisfaction with the results. smaller aggregate. 8. Ponding is not a desirable method of curing in an arid climate because of the quick drying that occurs when it is discontinued.Forms should have ample strength to sustain strong vibration of low slump concretes.2. they should be applied when the thoroughly wetted surface is still damp but no longer wet.2 Concrete mixes . (4. and curing will be interrupted or abbreviated (not to mention other less obvious items which influence the later appearance of unsightly cracks). the engineer should have the owner arrange for inspection by either the owner’s personnel. Philip D. the forms will provide adequate curing if exposed surfaces are protected from drying and provided they can be left in contact with the concrete for at least 7 days. Providing time for adjustment and gradual. Roger.. 627 pp. or a reliable professional inspection service who will insure that the construction is performed on the same basis as it was bid. Fadh H. (It is cautioned that too often. “Cracking in Fresh Concrete as Related to . When used on formed surfaces. the procedures discussed in this chapter can be used to produce a high quality concrete with the least probable amount of cracking. Cady. the mix proportioning should incorporate those factors that contribute to the lowest water content. if drying and thermal shrinkage cracking is to be prevented.Equipment (chutes. and bucket openings) should be capable of working effectively with lower slump. 8. finishing operations will be expedited with the water brush (or hose).1R to minimize or avoided. Rapid drying of the surfaces at the conclusion of the specified curing period should be 8. Without firm inspection and controls. Moreover.7 .. 2. E. When properly applied. No. Denver. Without the full and firm intent to confirm the specified character and degree of performance.These procedures should insure the presence of adequate moisture to sustain hydration and strength development in the surface concrete. pumps.1. At the end of the wet curing period.) Vibrators should be the largest and most powerful that can be operated in the placement.4 Finishing . 1975. When pumping is to be permitted and freedom from shrinkage cracking is important. 8. it is the responsibility of the engineer to develop effective designs and clear and specific specifications. conveyors. Formed grooves should be constructed in both sides of parapet. Dakhil.7 Curing and protection . Less than half the smooth grading curve amount of small coarse aggregate. sealing compounds are not adequate for thinner structural sections.5 or 19 mm). preferably at least 7 days. It is particularly important that flatwork joint grooves have a depth of at least l/5 of slab thickness. should be avoided during form removal. and a clear understanding of the job requirements by the contractor.S.6. more slump. the actions taken are those which increase drying shrinkage and resultant cracking: more sand.Conclusion As noted early in this chapter. slow elongation will minimize cracking.6 Contraction joints .6. 3. Upper lifts of formed concrete should be revibrated as late as the running vibrator will penetrate under its own weight.5 Forms . References 8. it is likely that concrete will contain more water than it should.6. 8. Thereafter. larger MSA concrete wherever it is appropriate and feasible to use. there is a serious chance that undesirable results will be obtained. This means: 1. Because drying is slow and prolonged. The lowest practical slump.6. hoppers.4 mm) deep. especially if it is crushed material. 1R for Cast-in-Place Concrete Guide to Joint Sealants for Concrete 504R Structures ASTM C 512 E 399 Concrete Test Method for Creep of Concrete in Compression Test Method for Plane-Strain Fracture Toughness of Metallic Materials Cornit Euro-International du B&ton and F&i&&m Internationale de la Prkcontrainte CEB-FIP Model Code for Concrete Structures The above publications may be obtained from the following organizations: American Association of State Highway and Transportation Officials 444 North Capital St. 8.” ACI JOURNAL.1. Suite 225 Washington.2R crete Standard Practice for the Use of Shrink223 age-compensating Concrete Guide for Concrete Floor and Slab Con302.1 tions for Normal. and 207.1R Effect of Restraint. Transpor304R tating. Volume Change. and Placing Concrete 305R Hot Weather Concreting Cold Weather Concreting 306R Standard Practice for Curing Concrete 308 Recommended Practice for Design and 313 Construction of Concrete Bins. No. and 1 abstained.W. 2 were not returned. 421-428.Recommended references The documents of the various standards producing organizations referred to in this document are listed below with their serial designation. Slabs.Cited references Cited references are provided at the end of each chapter.3R Accelerated Curing of Concrete at Atmospheric Pressure . 72. and Pile Caps (SP-17) Precast Concrete Units Used as Forms 347. It has been processed in accordance with the Institute procedure and is approved for publication and discussion. Placing and Finishing Steel Fiber Reinforced Chapter 9 . Brackets. and Bunkers for Storing Granular Materials Building Code Requirements for Rein318 forced Concrete 340.lR Design Handbook in Accordance with the Strength Design Method of ACI 318-83. Aug.2R 544.State of the Art Guide for Specifying. Box 19150 Detroit. Silos. DC 20001 American Concrete Institute P. 1975. 21 were affirmative. PA 19103 Cornit Euro-International du B&on and Federation Internationale de la Precontrainte . Proceedings V. N. Volume 1 .1R struction Guide for Measuring. pp. MI 48219 ASTM 1916 Race Street Philadelphia.224R-42 ACI COMMITTEE REPORT Reinforcement. Mixing. Footings. . Heavyweight.1R/ and Guide for Use of Admixtures in Con212.Beams.References 9. and Mass Concrete Admixtures for Concrete 212.O. 517..2R Reinforcement on Cracking of Massive Concrete Standard Practice for Selecting Propor211.2 .2R Mass Concrete 207. American Association of State Highway and Transportation Officials Plastic Fines in Graded Aggregate and T176 Soils By Use of the Sand Equivalent Test American Concrete Institute Guide to Durable Concrete 201.English edition available from: British Cement Association Wexham Springs Slough SL# 6PL ENGLAND 9. This report was submitted to letter ballot of the committee which consists of 24 members. Mixing. Liu Edward G. Liu J. Nawy Bernard L. Schrader Lewis H. Brander David Darwin* Fouad H. P. Yuan The committee voting on the 1990 revisions was as follows: Grant T. Meyers Past Chairman Robert E.CONTROL OF CRACKING 224R-43 ACI Committee 224 Cracking David Darwin Chairman R. Randall W. Bishara Roy W. Lloyd LeRoy Lutz V. Boggs Merle E. Barneyback. Poston Secretary Will Hansen Tony C. Tuthill Robert L. M. Eduardo Santos Basilio Alfred G. Ferry-Borges Peter Gergely Donald L. Tuthill* Thomas D. Kaar Tony C. Jr. Barth Alfred G. Philleo Milos Polivka Julius G. S. Halvorsen* Chairman Florian G. Price Ernest K. Verti Zenon Zielinski . Houghton Paul H. Schrader Wimal Suaris Lewis H. Everard J. Carlson Noel J. Nicholas Harry Palmbaum Arnfinn Rusten Andrew Scanlon Ernest K. Nawy John D. Fouad* Peter Gergely *Members contributing to these revisions. Malhotra Dan Naus Edward G. Bishara Howard L. Potyondy Robert E.